package/webkitgtk: add gobject-introspection optional dependency

Enable introspection when GObject Introspection is enabled.
Midori 9.0 requires webkitgtk built with gobject-introspection
support.

Signed-off-by: Adam Duskett <aduskett@gmail.com>
Reviewed-by: Adrian Perez de Castro <aperez@igalia.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>