[#32739] Add .gm-style for Google Maps

The current version of the Google Maps API uses .gm-style instead of #map_canvas or .google-maps, so the corresponding CSS lines need to be adapted.

JoomlaCode item: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_id=8103&tracker_item_id=32739