build: Lower gtk+-2.0 requirement to 2.20.0
build: Lower gtk+-2.0 requirement to 2.20.0

jku/rygel,Pelagicore/Media-Manager-Rygel-patched,GNOME/rygel,luisbg/rygel,jku/rygel,Tarnyko/rygel,Tarnyko/rygel,Pelagicore/Media-Manager-Rygel-patched,GNOME/rygel,Pelagicore/Media-Manager-Rygel-patched,GNOME/rygel