Fix for bug #3029.

The FreeType class does not override the ``get_extension`` method, by default this method will return None.

https://github.com/matplotlib/matplotlib/blob/6cf6063c7a3037d52545b2b7268623d33da0ecdd/setupext.py#L387

When calling the ``_check_for_pkg_config`` method a new extension will be created that what won't contain the original flags added by the ``add_flags`` method.

https://github.com/matplotlib/matplotlib/blob/6cf6063c7a3037d52545b2b7268623d33da0ecdd/setupext.py#L438