clear cache at enabling the module

When the module is installed, the plugin is not automatically discovered by the Feeds module.
The following code solves this issue. It will automatically clears the feeds plugin cache after enabling the module and the plugin will show up.

function feeds_xpathparser_enable() {
  cache_clear_all('plugins:feeds:plugins', 'cache');
}

thanks to @andrewlevine, see #862164: clear cache at installing the module