feedback-manager: Use guint for event id

The ID is inserted into a hashtable below with GUINT_TO_POINTER
and is set using self->next_id++ with next_id also being a guint,
so change the type accordingly.

src/fbd-feedback-manager.c

@@ -323,7 +323,7 @@ fbd_feedback_manager_handle_trigger_feedback (LfbGdbusFeedback      *object,
   FbdFeedbackManager *self;
   FbdEvent *event;
   GSList *feedbacks, *l;
-  gint event_id;
+  guint event_id;
   const gchar *sender;
   FbdFeedbackProfileLevel app_level, level, hint_level = FBD_FEEDBACK_PROFILE_LEVEL_FULL;
   gboolean found_fb = FALSE;