glusterd: make sure that mntent.h is conditionally included

Change-Id: I39c362c0908166707e10e8820cc1ee9a0989dcbe
BUG: 1089172
Signed-off-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-on: http://review.gluster.org/7584
Reviewed-by: Anand Avati <avati@redhat.com>
Tested-by: Anand Avati <avati@redhat.com>