librsvg: Only enable the Vala bindings if GObject Introspection is enabled

This avoids the following error:

  configure: error: Vala bindings require GObject Introspection

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>