Fix distcheck

With gdbus-testserver now being a compiled program, it no longer
lives in SRCDIR.

gio/tests/gdbus-threading.c

@@ -587,7 +587,7 @@ main (int   argc,
   session_bus_up ();
 
   /* this is safe; testserver will exit once the bus goes away */
-  g_assert (g_spawn_command_line_async (SRCDIR "/gdbus-testserver", NULL));
+  g_assert (g_spawn_command_line_async ("./gdbus-testserver", NULL));
 
   /* wait for the service to come up */
   usleep (500 * 1000);