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>
diff --git a/post/lib_powerpc/fpu/Makefile b/post/lib_powerpc/fpu/Makefile
index 556a833..5c2e804 100644
--- a/post/lib_powerpc/fpu/Makefile
+++ b/post/lib_powerpc/fpu/Makefile
@@ -19,5 +19,5 @@
 obj-y := $(objs-before-objcopy:.o=_.o)
 
 OBJCOPYFLAGS := -R .gnu.attributes
-$(obj)/%_.o: $(obj)/%.o
+$(obj)/%_.o: $(obj)/%.o FORCE
 	$(call if_changed,objcopy)