external/Makefile: fix build from fresh git clone.

```
make: *** No rule to make target 'external/gheap/gheap.h', needed by 'bitcoin/chainparams.o'.  Stop.
make: *** Waiting for unfinished jobs....
```

Just simplify the Makefile to make all the external headers we use
dependent on submodcheck.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

external/Makefile

@@ -16,13 +16,15 @@ endif
 LIBSODIUM_HEADERS := external/libsodium/src/libsodium/include/sodium.h
 LIBWALLY_HEADERS := external/libwally-core/include/wally_bip32.h		\
 			external/libwally-core/include/wally_core.h	\
+			external/libwally-core/include/wally_psbt.h	\
 			external/libwally-core/include/wally_crypto.h
 LIBSECP_HEADERS := external/libwally-core/src/secp256k1/include/secp256k1_ecdh.h		\
 		external/libwally-core/src/secp256k1/include/secp256k1.h
 JSMN_HEADERS := external/jsmn/jsmn.h
-GHEAP_HEADERS := external/gheap/gheap.h external/gheap/galgorithm.h
+GHEAP_HEADERS := external/gheap/gheap.h
+LIBBACKTRACE_HEADERS := external/libbacktrace/backtrace.h
 
-EXTERNAL_HEADERS := $(LIBSODIUM_HEADERS) $(LIBWALLY_HEADERS) $(LIBSECP_HEADERS) $(JSMN_HEADERS) $(GHEAP_HEADERS)
+EXTERNAL_HEADERS := $(LIBSODIUM_HEADERS) $(LIBWALLY_HEADERS) $(LIBSECP_HEADERS) $(JSMN_HEADERS) $(GHEAP_HEADERS) $(LIBBACKTRACE_HEADERS)
 EXTERNAL_LIBS := ${TARGET_DIR}/libwallycore.a ${TARGET_DIR}/libsecp256k1.a ${TARGET_DIR}/libjsmn.a ${TARGET_DIR}/libbacktrace.a
 
 EXTERNAL_INCLUDE_FLAGS :=					\
@@ -47,19 +49,17 @@ EXTERNAL_LDLIBS := -L${TARGET_DIR} $(patsubst lib%.a,-l%,$(notdir $(EXTERNAL_LIB
 submodcheck: FORCE
 	@tools/refresh-submodules.sh $(SUBMODULES)
 
+$(EXTERNAL_HEADERS): submodcheck
+
 # We build libsodium, since Ubuntu xenial has one too old.
 $(TARGET_DIR)/libsodium.a: $(TARGET_DIR)/libsodium-build/src/libsodium/libsodium.la
 	$(MAKE) -C $(TARGET_DIR)/libsodium-build DESTDIR=$$(pwd)/$(TARGET_DIR) install-exec
 
-external/libsodium/src/libsodium/include/sodium.h: submodcheck
-
 $(TARGET_DIR)/libsodium-build/src/libsodium/libsodium.la: external/libsodium/src/libsodium/include/sodium.h
 	cd external/libsodium && ./autogen.sh
 	mkdir -p ${TARGET_DIR}/libsodium-build
 	cd $(TARGET_DIR)/libsodium-build && $(TOP)/libsodium/configure CC="$(CC)" --enable-static=yes $(CROSSCOMPILE_OPTS) --enable-shared=no --prefix=/ --libdir=/ && $(MAKE)
 
-$(LIBWALLY_HEADERS) $(LIBSECP_HEADERS): submodcheck
-
 # libsecp included in libwally.
 # Wildcards here are magic.  See http://stackoverflow.com/questions/2973445/gnu-makefile-rule-generating-a-few-targets-from-a-single-source-file
 $(TARGET_DIR)/libsecp256k1.% $(TARGET_DIR)/libwallycore.%: $(TARGET_DIR)/libwally-core-build/src/secp256k1/libsecp256k1.la $(TARGET_DIR)/libwally-core-build/src/libwallycore.la
@@ -75,8 +75,6 @@ $(TARGET_DIR)/libwally-core-build/src/libwallycore.% $(TARGET_DIR)/libwally-core
 	mkdir -p ${TARGET_DIR}/libwally-core-build
 	cd ${TARGET_DIR}/libwally-core-build && PYTHON_VERSION=3 CFLAGS=-std=c99 ${TOP}/libwally-core/configure CC="$(CC)" --enable-static=yes $(CROSSCOMPILE_OPTS) --enable-module-recovery --enable-elements --enable-shared=no --prefix=/ --libdir=/ --enable-debug && $(MAKE)
 
-external/jsmn/jsmn.h: submodcheck
-
 # If we tell Make that the above builds both, it runs it twice in
 # parallel.  So we lie :(
 external/jsmn/jsmn.c: external/jsmn/jsmn.h
@@ -89,8 +87,6 @@ $(TARGET_DIR)/jsmn-build/jsmn.o: external/jsmn/jsmn.c Makefile
 $(TARGET_DIR)/libjsmn.a: $(TARGET_DIR)/jsmn-build/jsmn.o
 	$(AR) rc $@ $<
 
-external/libbacktrace/backtrace.h: submodcheck
-
 # Need separate build dir: changes inside submodule make git think it's dirty.
 $(TARGET_DIR)/libbacktrace.a: external/libbacktrace/backtrace.h
 	@mkdir $(TARGET_DIR)/libbacktrace-build 2>/dev/null || true