LU-6073 tests: use sys/xattr.h in tests/*.c

In lustre/tests/{multiop,orphan_linkea_check}.c include sys/xattr.h
rather than attr/xattr.h since the former is more likely to be
installed that the latter and provides the needed declarations.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: I79b49233ab42566c8084c2070d0d75a6508656dc
Reviewed-on: http://review.whamcloud.com/13197
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Tested-by: Jenkins
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Blake Caldwell <blakec@ornl.gov>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>