kbuild: Fix linking error built-in.o no such file or directory

This patch fixes the link error "built-in.o: no such file or directory".
The problem happens if "dirx/Makefile" contains only "obj-m += diry/
dirz/" and the empty "dirx/built-in.o" is missing. Adding $(subdir-m)
into check for builtin-target fixes this error.

Signed-off-by: Jiafu He <jay@goldhive.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>