Ignoring the models builder out of date flag (#2543)

The out of date models flag is automatically generated by the system, we don't want this in Git

Umbraco.gitignore

@@ -21,3 +21,6 @@
 
 # ImageProcessor DiskCache 
 **/App_Data/cache/
+
+# Ignore the Models Builder models out of date flag
+**/App_Data/Models/ood.flag