Backport !1931 “gs-plugin-appstream: Add missing silo read locker” to gnome-46
The refine thread accesses the self->silo, but does not hold the read locker, thus add it for thread safety.
Possibly related to a downstream report: https://bugzilla.redhat.com/show_bug.cgi?id=2272028
Trivial backport of !1931 (merged) to gnome-46.