kbuild: add -Werror=strict-prototypes flag unconditionally

-Wstrict-prototypes is added to KBUILD_CFLAGS first, then overridden
by -Werror=strict-prototypes later.

Let's add -Werror=strict-prototypes unconditionally because it is
supported by GCC 4.6, and also by Clang.

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