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.

Change-Id: I1e84969b26e87e36893b071103325a7a532ebbf9
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>