uapi/linux/target_core_user.h: fix headers_install.sh badness

scripts/headers_install.sh will transform __packed to
__attribute__((packed)), so the #ifndef is not necessary.
(and, in fact, it's problematic, because we'll end up with the header
 containing:
#ifndef __attribute__((packed))
#define __attribu...
and so forth.)

Cc: stable@vger.kernel.org # 3.18
Signed-off-by: Kyle McMartin <kyle@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>