LU-6401 header: remove libcfs.h in interval_tree.h

LU-6401 header: remove libcfs.h in interval_tree.h

The header libcfs.h for interval_tree.h is not needed
so we can just use the standard linux kernel headers
instead.h

Test-Parameters: trivial

Change-Id: I9c2c4f7e9bc227876818ff7213f392bdeb1112e0
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: https://review.whamcloud.com/24323
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Frank Zago <fzago@cray.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>