kbuild: add -Wdeclaration-after-statement flag unconditionally

We have raised the compiler requirement from time to time.
With commit cafa0010cd ("Raise the minimum required gcc version
to 4.6"), the minimum for GCC is 4.6 now.

This flag has been here since the pre-git era. It is documented in
the GCC 4.6 manual, and it is recognized by Clang and ICC as well.

Let's rip off the cc-option switch, and see if somebody complains
about it.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>

Makefile

@@ -807,7 +807,7 @@ endif
 NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include)
 
 # warn about C99 declaration after statement
-KBUILD_CFLAGS += $(call cc-option,-Wdeclaration-after-statement,)
+KBUILD_CFLAGS += -Wdeclaration-after-statement
 
 # disable pointer signed / unsigned warnings in gcc 4.0
 KBUILD_CFLAGS += -Wno-pointer-sign