kbuild: enforce -Werror=return-type

commit 172aad8 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Makefile

@@ -438,7 +438,7 @@ KBUILD_AFLAGS   := -D__ASSEMBLY__
 KBUILD_CFLAGS   := -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs \
 		   -fno-strict-aliasing -fno-common -fshort-wchar \
 		   -Werror-implicit-function-declaration \
-		   -Wno-format-security \
+		   -Werror=return-type -Wno-format-security \
 		   -std=gnu89
 KBUILD_CPPFLAGS := -D__KERNEL__
 KBUILD_AFLAGS_KERNEL :=