kbuild: disable clangs duplicate decl specifier warning

kbuild: disable clangs duplicate decl specifier warning

Disable the duplicate-decl-specifier warning. The warning is
triggered by every include of include/asm-generic/termios.h.

There is no easy way to get rid of this duplicate specifier:
One const specifier is generated by the typeof() call. The
type passed to the get_user macro is defined as const. But the
__get_user_check defines the type as const too. Since get_user
as well as the passed type is used in other contexts too, we
can not get rid of either const specifier easily...

Signed-off-by: Stefan Agner <stefan@agner.ch>