Fix for broken geocoding in mapviewer with OSM plugin

OSM Nominatim blocks user agents for default libraries.
This patch adds a specific user agent to the mapviewer example if
one isn't provided by the user

Change-Id: I9bc598eec6f53b0990c7cd0fb42cf3ef7807bedb
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>