s390x: only declare afs_sys_setgroups32_page where needed

s390x: only declare afs_sys_setgroups32_page where needed

Variable afs_sys_setgroups32_page is only used when __NR_setgroups32
is not defined.  Make the declaration depend on it as well to clear
up an unused variable warning.

Change-Id: I812b98d6ac4bf6ac923553ef3556c90d75d86fcb
Reviewed-on: http://gerrit.openafs.org/2600
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 4129b055a6236e892e1dc2c5d2023d1947495811)
Reviewed-on: http://gerrit.openafs.org/2672