[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>
Not any good / non-invasive ideas (it only happens when we don't have pk-rpm-db-clean, which for recent OS versions we should always have.