as-app: properly initialize unique_id_mutex
as-app: properly initialize unique_id_mutex

We are using it, clearing it, but e never initialize it.
For some reasons it seems to work, not sure how or why,
but when glib is uilt with clang, then the problem
surfaces and we get a nice segmentation fault.
Properly initializing it fixes this

Signed-off-by: Marc-Antoine Perennou <07f76cf0511c79b361712839686f3cee8c75791c@Perennou.com>

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