linux-kernel-module: move keyring-specific

function afs_set_session_keyring into
if defined(LINUX_KEYRING_SUPPORT)
block. Otherwise compilation fails.

Change-Id: I44bb015990782793eac9326b983b704b2248b230
Reviewed-on: http://gerrit.openafs.org/10324
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>