MIPS: Fix siginfo.h to use strict posix types

Commit 85efde6f ("make exported headers use strict posix types")
changed the asm-generic siginfo.h to use the __kernel_* types, and
commit 3a471cbc ("remove __KERNEL_STRICT_NAMES") make the internal
types accessible only to the kernel, but the MIPS implementation hasn't
been updated to match.

Switch to proper types now so that the exported asm/siginfo.h won't
produce quite so many compiler errors when included alone by a user
program.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Christopher Ferris <cferris@google.com>
Cc: linux-mips@linux-mips.org
Cc: <stable@vger.kernel.org> # 2.6.30-
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/12477/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>