devtools/Makefile: add devtools/onion.c to DEVTOOLS_TOOL_SRC

This way the object file correctly depends on external headers.  Currently
a parallel build on a clean tree can give:

```
In file included from ./common/sphinx.h:6:0,
                 from devtools/onion.c:5:
./bitcoin/pubkey.h:8:10: fatal error: secp256k1.h: No such file or directory
 #include <secp256k1.h>
          ^~~~~~~~~~~~~
compilation terminated.
<builtin>: recipe for target 'devtools/onion.o' failed
```

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

devtools/Makefile

@@ -1,6 +1,6 @@
 DEVTOOLS_SRC := devtools/gen_print_wire.c devtools/print_wire.c
 DEVTOOLS_OBJS := $(DEVTOOLS_SRC:.c=.o)
-DEVTOOLS_TOOL_SRC := devtools/bolt11-cli.c devtools/decodemsg.c
+DEVTOOLS_TOOL_SRC := devtools/bolt11-cli.c devtools/decodemsg.c devtools/onion.c
 DEVTOOLS_TOOL_OBJS := $(DEVTOOLS_TOOL_SRC:.c=.o)
 
 DEVTOOLS_COMMON_OBJS :=				\