base-manager: automatic required subsystem detection

Instead of assuming we require a fixed set of subsystems to monitor,
compile the full list based on what the plugins have requested
themselves.

src/mm-plugin.h

@@ -126,6 +126,7 @@ GType mm_plugin_get_type (void);
 G_DEFINE_AUTOPTR_CLEANUP_FUNC (MMPlugin, g_object_unref)
 
 const gchar           *mm_plugin_get_name                (MMPlugin *self);
+const gchar          **mm_plugin_get_allowed_subsystems  (MMPlugin *self);
 const gchar          **mm_plugin_get_allowed_udev_tags   (MMPlugin *self);
 const guint16         *mm_plugin_get_allowed_vendor_ids  (MMPlugin *self);
 const mm_uint16_pair  *mm_plugin_get_allowed_product_ids (MMPlugin *self);