ptrace: remove linux/compat.h inclusion under CONFIG_COMPAT

Commit 84c751bd ("ptrace: add ability to retrieve signals without
removing from a queue (v4)") includes <linux/compat.h> globally in
ptrace.c

This patch removes inclusion under if defined CONFIG_COMPAT.

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>