kbuild: specify FORCE in Makefile.headersinst as .PHONY target

Swap the order of ".PHONY: $(PHONY)" and "PHONY += FORCE"
so that FORCE is correctly specified as a .PHONY target.

Use a preferred way for specifying $(subdirs) as .PHONY targets.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>