spec: Let translations be properly updated

Libvirt tarball contains po/stamp-po file which prevents any po/*.gmo
file to be regenerated even if a corresponding po/*.po file is newer. By
removing the stamp-po file, all *.gmo files are properly updated if
required. This allows downstreams to provide patches that update
translations.

libvirt.spec.in

@@ -1421,6 +1421,7 @@ driver
  autoreconf -if
 %endif
 
+rm -f po/stamp-po
 %configure %{?_without_xen} \
            %{?_without_qemu} \
            %{?_without_openvz} \