This adds ".inc.php" files to the included overrides while maintaining
support for existing ".inc" files previously included via overrides.
Roundcube itself uses "inc.php" files and these overrides are expected
to match that format. Switching to "inc.php" both tells the user that
these need to be proper php files and conveys they are used for changing
the same settings that Roundcube's inc.php files modify.
