Kbuild: make designated_init attribute fatal

If a structure is marked with __attribute__((designated_init)) from
GCC or Sparse, it needs to have all static initializers using designated
initialization. Fail the build for any missing cases. This attribute will
be used by the randstruct plugin to make sure randomized structures are
being correctly initialized.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>