(bug 44214) Fix bundled GeSHi library inclusion path

Currently, the SyntaxHighlight GeSHi extension includes geshi/geshi.php.

This creates a conflict when an up to date GeSHI library exists in PHP included path.

This change let the SyntaxHighlight GeSHi include our bundled version instead.

Change-Id: Ie8f9aa6182a38508201d639723e876c156d8b0d1