Add libxslt-dev and libxml2-dev libraries to linux nodes

Libxslt is used to implement support for client side xslt in JavaScript.
Without Libxslt there is no support in WebKit.

Libxml2 is used for XML parsing

Change-Id: Ica8a2de88cd5ba7275dc77e1405154bd15ac1342
Reviewed-by: Tony Sarajärvi <tony.sarajarvi@digia.com>