packaging: added leading question mark to dist tag

when using %{dist} to mark a package as having been built for a
specific distribution, we need to add it to Release field with a
leading question mark.

Change-Id: I1a2ea23df3cb73969d698440e26a9e4e861e2a9a
Bug-Url: https://bugzilla.redhat.com/1094024
Signed-off-by: Yaniv Dary <ydary@redhat.com>