LU-8130 libcfs: don't include rhashtable if unavailable

LU-8130 libcfs: don't include rhashtable if unavailable

Don't include <linux/rhashtable.h> if it is not available.

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I80b2ee63fb2a438399359f8052a5063429dd6506
Reviewed-on: https://review.whamcloud.com/34020
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Ben Evans <bevans@cray.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>