Fix missing initializer warning in gio/tests/gdbus-peer-object-manager.c

gio/tests/gdbus-peer-object-manager.c: In function ‘mock_interface_get_vtable’:
gio/tests/gdbus-peer-object-manager.c:111:3: error: missing initializer for field ‘padding’ of ‘GDBusInterfaceVTable’ {aka ‘struct _GDBusInterfaceVTable’}
  111 |   };
      |   ^

gio/tests/gdbus-peer-object-manager.c

@@ -108,6 +108,7 @@ mock_interface_get_vtable (GDBusInterfaceSkeleton *interface)
     NULL,
     mock_interface_get_property,
     NULL,
+    { 0 }
   };
 
   return &vtable;