[rpmdb] request pk-rpm-db-clean to rebuild db if it exists

/usr/libexec/pk-rpm-db-clean rebuilds rpm db on boot automatically if
/var/lib/PackageKit/scheduled-rebuilddb exists, so ask it to do the
job if it is possible

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
Why not `if [ -f /usr/libexec/pk-rpm-db-clean ]`? The `ls` command here will have a side-effect (either showing an error message or printing out the filename when it's found).