win32: stop leaking GPrivate data

Instead of running the GPrivate destructors from our thread proxy code,
run it from the DllMain handler for the DLL_THREAD_DETACH case.

This should ensure that thread-local data is free at the exit of all
threads -- not just the ones we created for ourselves.

https://bugzilla.gnome.org/show_bug.cgi?id=660745

glib/glib-init.c

@@ -225,6 +225,10 @@ DllMain (HINSTANCE hinstDLL,
       glib_init ();
       break;
 
+    case DLL_THREAD_DETACH:
+      g_thread_win32_thread_detach ();
+      break;
+
     default:
       /* do nothing */
       ;