kbuild: add -Wdeclaration-after-statement flag unconditionally

We have raised the compiler requirement from time to time.
With commit cafa0010 ("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>