rpm spec file: Don't overwrite user-modified /etc/stbt/stbt.conf

From the Fedora RPM Guide[1]:

> If you use %config(noreplace), the file will not overwrite an existing
> file that has been modified. If the file has not been modified on disk,
> the rpm command will overwrite the file. But, if the file has been
> modified on disk, the rpm command will copy the new file with an extra
> file-name extension of .rpmnew.

[1] http://docs.fedoraproject.org/en-US/Fedora_Draft_Documentation/0.1/html/RPM_Guide/ch09s05s03.html