kbuild: post: fix dependency tracking correctly

$(call if_changed,...) must take FORCE as a prerequite.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>

post/lib_powerpc/fpu/Makefile

@@ -19,5 +19,5 @@ ccflags-y := -mhard-float -fkeep-inline-functions
 obj-y := $(objs-before-objcopy:.o=_.o)
 
 OBJCOPYFLAGS := -R .gnu.attributes
-$(obj)/%_.o: $(obj)/%.o
+$(obj)/%_.o: $(obj)/%.o FORCE
 	$(call if_changed,objcopy)