Linux: always include <linux/uidgid.h> headerfile

when it is required. In some linux-kernels (like in SLES11 SP3)
it is not done automatically and the compilation fails.

Reviewed-on: http://gerrit.openafs.org/10471
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 779ab18bafdea3535b403ba62f3ec8a8b127748e)

Change-Id: If5bc31e0cf09634f5d36bbe7757919b6f1dc310f
Reviewed-on: http://gerrit.openafs.org/10472
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>