plugins/Makefile: Only link in libplugin-pay for plugins that need it.

Changelog-None: cleanup only.

Before this change:

```
$ ls -l plugins/bcli
-rwxrwxr-x 1 zmnscpxj zmnscpxj 1914976 Aug  5 21:54 plugins/bcli
```

After this change:

```
$ ls -l plugins/bcli
-rwxrwxr-x 1 zmnscpxj zmnscpxj 1830552 Aug  5 22:00 plugins/bcli
```

We already duplicate a lot of code between `lightningd` and eeach
builtin plugin because we do not use .so for `common/`, but including
an object file with code that is not referenced in the executable is
fairly low-hanging size optimization fruit.

plugins/Makefile

@@ -13,19 +13,25 @@ PLUGIN_BCLI_OBJS := $(PLUGIN_BCLI_SRC:.c=.o)
 PLUGIN_KEYSEND_SRC := plugins/keysend.c
 PLUGIN_KEYSEND_OBJS := $(PLUGIN_KEYSEND_SRC:.c=.o)
 
-PLUGIN_LIB_SRC := plugins/libplugin.c plugins/libplugin-pay.c
-PLUGIN_LIB_HEADER := plugins/libplugin.h plugins/libplugin-pay.h
+PLUGIN_LIB_SRC := plugins/libplugin.c
+PLUGIN_LIB_HEADER := plugins/libplugin.h
 PLUGIN_LIB_OBJS := $(PLUGIN_LIB_SRC:.c=.o)
 
+PLUGIN_PAY_LIB_SRC := plugins/libplugin-pay.c
+PLUGIN_PAY_LIB_HEADER := plugins/libplugin-pay.h
+PLUGIN_PAY_LIB_OBJS := $(PLUGIN_PAY_LIB_SRC:.c=.o)
+
 PLUGIN_ALL_SRC :=				\
 	$(PLUGIN_AUTOCLEAN_SRC)			\
 	$(PLUGIN_BCLI_SRC)			\
 	$(PLUGIN_FUNDCHANNEL_SRC)		\
 	$(PLUGIN_KEYSEND_SRC)			\
 	$(PLUGIN_LIB_SRC)			\
+	$(PLUGIN_PAY_LIB_SRC)			\
 	$(PLUGIN_PAY_SRC)
 PLUGIN_ALL_HEADER :=				\
-	$(PLUGIN_LIB_HEADER)
+	$(PLUGIN_LIB_HEADER)			\
+	$(PLUGIN_PAY_LIB_HEADER)
 PLUGIN_ALL_OBJS := $(PLUGIN_ALL_SRC:.c=.o)
 
 PLUGINS :=					\
@@ -74,7 +80,8 @@ PLUGIN_COMMON_OBJS :=				\
 	wire/tlvstream.o			\
 	wire/towire.o
 
-plugins/pay: bitcoin/chainparams.o $(PLUGIN_PAY_OBJS) $(PLUGIN_LIB_OBJS) $(PLUGIN_COMMON_OBJS) $(JSMN_OBJS) $(CCAN_OBJS)
+plugins/pay: bitcoin/chainparams.o $(PLUGIN_PAY_OBJS) $(PLUGIN_LIB_OBJS) $(PLUGIN_PAY_LIB_OBJS) $(PLUGIN_COMMON_OBJS) $(JSMN_OBJS) $(CCAN_OBJS)
+$(PLUGIN_PAY_OBJS): $(PLUGIN_PAY_LIB_HEADER)
 
 plugins/autoclean: bitcoin/chainparams.o $(PLUGIN_AUTOCLEAN_OBJS) $(PLUGIN_LIB_OBJS) $(PLUGIN_COMMON_OBJS) $(JSMN_OBJS) $(CCAN_OBJS)
 
@@ -82,7 +89,8 @@ plugins/fundchannel: common/addr.o $(PLUGIN_FUNDCHANNEL_OBJS) $(PLUGIN_LIB_OBJS)
 
 plugins/bcli: bitcoin/chainparams.o $(PLUGIN_BCLI_OBJS) $(PLUGIN_LIB_OBJS) $(PLUGIN_COMMON_OBJS) $(JSMN_OBJS) $(CCAN_OBJS)
 
-plugins/keysend: bitcoin/chainparams.o wire/tlvstream.o wire/gen_onion_wire.o $(PLUGIN_KEYSEND_OBJS) $(PLUGIN_LIB_OBJS) $(PLUGIN_COMMON_OBJS) $(JSMN_OBJS) $(CCAN_OBJS)
+plugins/keysend: bitcoin/chainparams.o wire/tlvstream.o wire/gen_onion_wire.o $(PLUGIN_KEYSEND_OBJS) $(PLUGIN_LIB_OBJS) $(PLUGIN_PAY_LIB_OBJS) $(PLUGIN_COMMON_OBJS) $(JSMN_OBJS) $(CCAN_OBJS)
+$(PLUGIN_KEYSEND_OBJS): $(PLUGIN_PAY_LIB_HEADER)
 
 $(PLUGIN_ALL_OBJS): $(PLUGIN_LIB_HEADER)