gnu: Add gtksourceview.

* gnu/packages/gtk.scm (gtksourceview): Rename to...
  (gtksourceview-2): ... this.
  (gtksourceview): Update to 3.18.1.
* gnu/packages/ocaml.scm (lablgtk)[inputs]: Replace "gtksourceview" with "gtksourceview-2".

Co-authored-by: 宋文武 <iyzsong@gmail.com>