Makefile: add dependency to fix parallel build.

```
cc tools/headerversions.c
tools/headerversions.c:3:10: fatal error: 'config.h' file not found
#include "config.h"
         ^~~~~~~~~~
yes
1 error generated.
make: *** [Makefile:299: tools/headerversions.o] Error 1
```

Signed-off-by: Rusty Russell <[email protected]>

tools/Makefile

@@ -12,7 +12,7 @@ TOOLS_COMMON_OBJS = common/utils.o
 
 # We force make to relink this every time, to detect version changes.
 # Do it atomically, otherwise parallel builds can get upset!
-tools/headerversions: $(FORCE) tools/headerversions.o libccan.a
+tools/headerversions: $(FORCE) tools/headerversions.o libccan.a ccan/config.h
 	@trap "rm -f $@.tmp.$$$$" EXIT; $(LINK.o) tools/headerversions.o libccan.a $(LOADLIBES) $(LDLIBS) -o $@.tmp.$$$$ && mv $@.tmp.$$$$ $@
 
 tools/check-bolt: tools/check-bolt.o $(TOOLS_COMMON_OBJS)