Fix missing initializer warning in gio/gtask.c

gio/gtask.c:2153:1: error: missing initializer for field ‘closure_callback’ of ‘GSourceFuncs’ {aka ‘struct _GSourceFuncs’}
 2153 | };
      | ^

gio/gtask.c

@@ -2162,7 +2162,9 @@ GSourceFuncs trivial_source_funcs = {
   NULL, /* prepare */
   NULL, /* check */
   trivial_source_dispatch,
-  NULL
+  NULL, /* finalize */
+  NULL, /* closure */
+  NULL  /* marshal */
 };
 
 static void