staging/lustre/build: fix compilation issue with is_compat_task

After removing LIBCFS_HAVE_IS_COMPAT_TASK test we have a
compilation issue with kernels configured without CONFIG_COMPAT.

Lustre-change: http://review.whamcloud.com/7118
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-2800


Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Peng Tao <bergwolf@gmail.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>