kbuild: fix single directory build

Commit f110e5a2 ("kbuild: refactor single builds of *.ko") was wrong.

KBUILD_MODULES _is_ needed for single builds.

Otherwise, "make foo/bar/baz/" does not build module objects at all.

Fixes: f110e5a2 ("kbuild: refactor single builds of *.ko")
Reported-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: David Sterba <dsterba@suse.com>

Makefile

@@ -1978,6 +1978,8 @@ endif
 
 single-goals := $(addprefix $(build-dir)/, $(single-no-ko))
 
+KBUILD_MODULES := 1
+
 endif
 
 # Preset locale variables to speed up the build process. Limit locale