There is already an abstraction in libgdx that works on all current supported platforms (Android, Desktop, HTML5, and iOS aswell).
Gdx.net.openURI("http://stackoverflow.com/questions/21242116/opening-urls-with-the-browser-robovm-ios");
(This is the implementation of RoboVM: RoboVM IOSNet#openURI just as a reference, use the abstraction. No need to make platform specific code when libgdx already did it for you ;) )