audit: use uapi/linux/audit.h for AUDIT_ARCH declarations

The syscall.h headers were including linux/audit.h but really only
needed the uapi/linux/audit.h to get the requisite defines.  Switch to
the uapi headers.

Signed-off-by: Eric Paris <eparis@redhat.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-mips@linux-mips.org
Cc: linux-s390@vger.kernel.org
Cc: x86@kernel.org