kbuild: enforce -Werror=return-type

Catch errors which at least gcc tolerates by default:
 warning: 'return' with no value, in function returning non-void [-Wreturn-type]

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>