mm/process_vm_access.c: include compat.h

Fix the build error:

  mm/process_vm_access.c:277:5: error: implicit declaration of function 'in_compat_syscall'; did you mean 'in_ia32_syscall'? [-Werror=implicit-function-declaration]

Fixes: 38dc5079

 "Fix compat regression in process_vm_rw()"
Reported-by:  <syzbot+5b0d0de84d6c65b8dd2b@syzkaller.appspotmail.com>
Cc: Kyle Huey <me@kylehuey.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Hellwig <hch@lst.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>