Linux: Fix prototypes for afs_xsetgroups and sys_setgroupsp and friends

Linux: Fix prototypes for afs_xsetgroups and sys_setgroupsp and friends

Fixes some “warning: function declaration isn’t a prototype”, and
makes the prototypes consistent with the corresponding definitions.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Reviewed-on: http://gerrit.openafs.org/3278
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit c07b47169adb85621c328a3687ea62ef84e43bb1)

Change-Id: Ia91a598fc4b25f5fd7f29aed38a7caa3437ffc89
Reviewed-on: http://gerrit.openafs.org/3490
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>