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

Makefile

@@ -497,7 +497,7 @@ KBUILD_AFLAGS   := -D__ASSEMBLY__ -fno-PIE
 KBUILD_CFLAGS   := -Wall -Wundef -Werror=strict-prototypes -Wno-trigraphs \
 		   -fno-strict-aliasing -fno-common -fshort-wchar -fno-PIE \
 		   -Werror=implicit-function-declaration -Werror=implicit-int \
-		   -Wno-format-security \
+		   -Werror=return-type -Wno-format-security \
 		   -std=gnu89
 KBUILD_CPPFLAGS := -D__KERNEL__
 KBUILD_AFLAGS_KERNEL :=