Set missing license field

Author: jmid

packages/conf-gtk3/conf-gtk3.18/opam

@@ -1,6 +1,7 @@
 opam-version: "2.0"
 maintainer: "[email protected]"
 authors: "The GTK Toolkit"
+license: "LGPL-2.1-or-later"
 homepage: "https://developer.gnome.org/"
 bug-reports: "https://github.com/garrigue/lablgtk/issues"
 build: [