rpm: Really undefine %dist

rpm: Really undefine %dist

rpmbuild --eval '%undefine dist' wasn't sticking on redhat 5.x. I think
the correct way to undefine this is to --define 'dist %undefined'

Some redhat 5.x installs have %dist defined depending on what else has been
installed.

Reviewed-on: http://gerrit.openafs.org/4643
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 0b983054f88d69ca4c0cda07e94103f216bd881d)

Change-Id: I41828880a10097059ae0297ae191fa2a96e804d2
Reviewed-on: http://gerrit.openafs.org/4665
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>