kbuild: fix make dir/file.xx when asm symlink is missing

Added a dependency so we do full preparation before trying to build single
file targets. This fixes a case where Andrew Morton did:
	make kernel/sched.o
        rm include/asm
	make kernel/sched.o     -> splat

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
1 file changed