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: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>