rxkad: ticket5.c fix typo in #if statement

rxkad: ticket5.c fix typo in #if statement

commit 98ca332c4a5ac9e5687fb4fe21b350134bc74d1b (rxkad: v5der.c format
truncation warnings) contains a typo in the test for clang (_clang
instead of __clang__)

Correct the typo in the #if statement to test for __clang__

Reviewed-on: https://gerrit.openafs.org/13754
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit e3dbd8a5886734f6390126e155cc259b0de5af51)

Change-Id: I6c2a9851e668947c61d97e013c84808f738c4e52
Reviewed-on: https://gerrit.openafs.org/13756
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>