Include linux/freezer.h in rx_kmutex.c

Include linux/freezer.h in rx_kmutex.c

rx_kmutex.c calls refrigerator(), which is prototyped in linux/freezer.h,
but was not including it, causing build failures on Linux 2.6.32-5
(Debian).

Change-Id: I3867be6e7b8478d6fab8b97a4c923f979029517e
Reviewed-on: http://gerrit.openafs.org/2380
Tested-by: Russ Allbery <rra@stanford.edu>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>