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.

Lustre-change: https://review.whamcloud.com/34020
Lustre-commit: 29d627f860bc1963f2103ea441577dbd18d71344

Fixes: ac8d93c9f6f9 ("LU-8130 libcfs: support latest rhashtable API")

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I80b2ee63fb2a438399359f8052a5063429254035
Reviewed-by: Ben Evans <bevans@cray.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/35565
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>