build: Remove dummy gi-docgen dependency

Instead, add a provide to the wrap that allows it to be pulled in
by the `find_program('gi-docgen')` call.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3131>

subprojects/gi-docgen.wrap

@@ -4,3 +4,6 @@ url=https://gitlab.gnome.org/GNOME/gi-docgen.git
 push-url=ssh://git@gitlab.gnome.org:GNOME/gi-docgen.git
 revision=main
 depth=1
+
+[provide]
+program_names = gi-docgen