Fix crash on mouse server mode grab with gtk2
Fix crash on mouse server mode grab with gtk2

With gtk3 gdk_cursor_unref = g_object_unref. But not with older
version. Let's be consitent with the rest of the code and use
cursor_unref().

SPICE/spice-gtk,guodong/spice-gtk,elmarco/spice-gtk,guodong/spice-gtk,fgouget/spice-gtk,mathslinux/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,mathslinux/spice-gtk,dezelin/spice-gtk,elmarco/spice-gtk,fgouget/spice-gtk,flexVDI/spice-gtk,SPICE/spice-gtk,guodong/spice-gtk,dezelin/spice-gtk,flexVDI/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,Fantu/spice-gtk,SPICE/spice-gtk,SPICE/spice-gtk,Fantu/spice-gtk,fgouget/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,elmarco/spice-gtk,mathslinux/spice-gtk,guodong/spice-gtk,elmarco/spice-gtk,mathslinux/spice-gtk,dezelin/spice-gtk,Fantu/spice-gtk,Fantu/spice-gtk,dezelin/spice-gtk