[amiwm] add a missing Makefile depedency

This isn't completely enough to make this build with gmake -j, but it's
one of many things that are missing.

Makefile.in

@@ -66,7 +66,7 @@ lib_clean:
 menu.o: menu.c
 	$(CC) -c $(CPPFLAGS) $(ALL_CFLAGS) -DAMIWM_HOME=\"$(AMIWM_HOME)\" $<
 
-rc.o: rc.c
+rc.o: rc.c gram.h
 	$(CC) -c $(CPPFLAGS) $(ALL_CFLAGS) -DAMIWM_HOME=\"$(AMIWM_HOME)\" $<
 
 diskobject.o: diskobject.c