rpmbuild.sh: git builds count as pre-release of the next version

This avoids the situation where a newer RPM release has been built (e.g.
0.15.1-2) which compares higher than the RPM EVR for git builds
(0.15.1-1.git.123.abcdef).

Change-Id: I20a53260610fb477390aed9f074609d7c503a6ac