Change license to GPLv3 so we can depend on FreeRDP
Freerdp's license is imcompatible with GPLv2 (Boxes current license). Since commit e6088aea we are having gtk-frdp as a submodule of our project. Gtk-frdp is a gobject wrapper around the freerdp API, and therefore we are depending on freerdp.
Freerdp has an Apache v2.0 license, see https://github.com/FreeRDP/FreeRDP/blob/master/LICENSE
See also "Apache License v2.0 and GPL Compatibility" https://www.apache.org/licenses/GPL-compatibility.html