fix include order in sys-i386/registers.c

We want sys/ptrace.h before any includes of linux/ptrace.h and
asm/user.h pulls the latter.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>