MIPS: ptrace.h: Add a missing include

Commit a79ebea6 (MIPS: ptrace: Fix user pt_regs definition,
use in ptrace_{get, set}regs()) converted struct pt_regs to use __u64.
Some userspace applications (e.g. GDB) include this file directly,
and fail to see this type. Fix by including <linux/types.h>.

The patch fixes the following build failure with GDB 7.8 when using
GLIBC headers created against Linux 3.17:

In file included from /home/aaro/los/work/shared/gdb-7.8/gdb/mips-linux-nat.c:37:0:
/home/aaro/los/work/mips/rootfs/mips-linux-gnu/usr/include/asm/ptrace.h:32:2: error: unknown type name '__u64'
  __u64 regs[32];
  ^
/home/aaro/los/work/mips/rootfs/mips-linux-gnu/usr/include/asm/ptrace.h:35:2: error: unknown type name '__u64'
  __u64 lo;
  ^
/home/aaro/los/work/mips/rootfs/mips-linux-gnu/usr/include/asm/ptrace.h:36:2: error: unknown type name '__u64'
  __u64 hi;
  ^

Fixes: a79ebea6 ("MIPS: ptrace: Fix user pt_regs definition, use in ptrace_{get, set}regs()")
Cc: stable@vger.kernel.org # 3.17
Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: Alex Smith <alex@alex-smith.me.uk>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/8067/


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