LU-2800 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.
The fix for Change-Id: Ieffc3b32611e00f2a19aa2645205ec6e1cf57a20

Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Change-Id: I2725e892aea1a86bd43fe19ab099f6f116d1fdd8
Reviewed-on: http://review.whamcloud.com/7118
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Tested-by: Hudson
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>