kbuild: add -Wvla flag unconditionally

This flag is documented in the GCC 4.6 manual, and recognized by
Clang as well. Let's rip off the cc-option switch.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Sedat Dilek <sedat.dilek@gmail.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Acked-by: Kees Cook <keescook@chromium.org>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>

Makefile

@@ -841,7 +841,7 @@ NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include)
 KBUILD_CFLAGS += -Wdeclaration-after-statement
 
 # Variable Length Arrays (VLAs) should not be used anywhere in the kernel
-KBUILD_CFLAGS += $(call cc-option,-Wvla)
+KBUILD_CFLAGS += -Wvla
 
 # disable pointer signed / unsigned warnings in gcc 4.0
 KBUILD_CFLAGS += -Wno-pointer-sign