glib.supp: Suppress g_task -> thread leaks

There are variants of this, but right now because `GTask` has its
own pool which isn't necessarily cleaned up on exit, we need
to suppress all of this.

Closes: #410
Approved by: giuseppe

tests/glib.supp

@@ -400,6 +400,14 @@
    fun:g_thread_pool_push
    fun:g_task_start_task_thread
 }
+{
+   g_task_to_pool
+   Memcheck:Leak
+   ...
+   fun:g_thread_pool_start_thread
+   ...
+   fun:g_task_run_in_thread
+}
 {
    g_get_language_names
    Memcheck:Leak