Fix compile error for PPC64 gcc 6.1.1

Fix compile error for PPC64 gcc 6.1.1

Cast function pointer stubs to remove compile errors
on Fedora 24 PPC64 with ggcc 6.1.1

FIXES 133407

Reviewed-on: https://gerrit.openafs.org/12386
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 733dcec01784617e3354c2b8b29f50b09464a4bb)

Change-Id: Ic3af05a33a9df00449c5d68aa0c481167c23e60a
Reviewed-on: https://gerrit.openafs.org/12388
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>