LU-7394 obd: remove HAVE_SELINUX_IS_ENABLED

LU-7394 obd: remove HAVE_SELINUX_IS_ENABLED

In obd_mount_server.c unconditionally include linux/selinux.h. Include
linux/selinux.h in xattr_security.c.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: Iea7a64d4e66d4c78494da40642111085b95700f9
Reviewed-on: http://review.whamcloud.com/17054
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>