Add a destructor for AsRefString
Add a destructor for AsRefString

AsRefString uses a singleton hash table for deduplicating strings. This
is all "possibly lost" memory according to valgrind as the hash table
never gets destroyed.

This commit adds a custom destructor so that we can clean up the
singleton hash table as well and avoid unnecessary noise in valgrind
output.

ximion/appstream-glib,hughsie/appstream-glib,ximion/appstream-glib,ximion/appstream-glib,hughsie/appstream-glib,hughsie/appstream-glib