scripts/gdb: rebuild constants.py on dependancy change

The autogenerated constants.py file was only being built on the initial
call, and if the constants.py.in file changed.  As we are utilising the
CPP hooks, we can successfully use the call if_changed_dep rules to
determine when to rebuild the file based on it's inclusions.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Kieran Bingham <[email protected]>
Reported-by: Jan Kiszka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

scripts/gdb/linux/Makefile

@@ -13,8 +13,9 @@ quiet_cmd_gen_constants_py = GEN     $@
 	$(CPP) -E -x c -P $(c_flags) $< > $@ ;\
 	sed -i '1,/<!-- end-c-headers -->/d;' $@
 
-$(obj)/constants.py: $(SRCTREE)/$(obj)/constants.py.in
-	$(call if_changed,gen_constants_py)
+targets += constants.py
+$(obj)/constants.py: $(SRCTREE)/$(obj)/constants.py.in FORCE
+	$(call if_changed_dep,gen_constants_py)
 
 build_constants_py: $(obj)/constants.py
 	@: