tidy up warning

warning: incompatible pointer types passing 'ObjModule *' to parameter of type 'Obj *'

src/vm/wren_value.c

@@ -1090,7 +1090,7 @@ static void blackenFn(WrenVM* vm, ObjFn* fn)
   wrenGrayBuffer(vm, &fn->constants);
 
   // Mark the module it belongs to, in case it's been unloaded.
-  wrenGrayObj(vm, fn->module);
+  wrenGrayObj(vm, (Obj*)fn->module);
 
   // Keep track of how much memory is still in use.
   vm->bytesAllocated += sizeof(ObjFn);