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.

Change-Id: I9ad6854a337c2085bb2db5e5f98e7d2f9889c4e4
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>