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

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

In lustre/tests/mpi/mdsrate.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: I3b3f6a2cc2a5a7153bb3dbff535d7c1b3472f9b8
Reviewed-on: http://review.whamcloud.com/15391
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>