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>

scripts/Makefile.build

@@ -82,7 +82,7 @@ ifneq ($(strip $(lib-y) $(lib-m) $(lib-n) $(lib-)),)
 lib-target := $(obj)/lib.a
 endif
 
-ifneq ($(strip $(obj-y) $(obj-m) $(obj-n) $(obj-) $(lib-target)),)
+ifneq ($(strip $(obj-y) $(obj-m) $(obj-n) $(obj-) $(subdir-m) $(lib-target)),)
 builtin-target := $(obj)/built-in.o
 endif