kbuild: specify build_docproc as a phony target

PHONY target is more suitable for "build_docproc" target.

Because PHONY targets are always executed, they do not
have to take FORCE as a prerequisite.
Signed-off-by: NMasahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: NMichal Marek <mmarek@suse.cz>

scripts/Makefile

@@ -27,10 +27,10 @@ always		:= $(hostprogs-y) $(hostprogs-m)
 hostprogs-y += unifdef docproc
 
 # These targets are used internally to avoid "is up to date" messages
-PHONY += build_unifdef
-build_unifdef: scripts/unifdef FORCE
+PHONY += build_unifdef build_docproc
+build_unifdef: $(obj)/unifdef
 	@:
-build_docproc: scripts/docproc FORCE
+build_docproc: $(obj)/docproc
 	@:
 
 subdir-$(CONFIG_MODVERSIONS) += genksyms