package/libvorbis: add an upstream URL to Config.in

Add an upstream URL to the help text in Config.in. This
addresses the 'Missing' URL status in the package stats
web page output.

Signed-off-by: Mark Corbin <mark@dibsco.co.uk>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libvorbis/Config.in

@@ -13,3 +13,5 @@ config BR2_PACKAGE_LIBVORBIS
 	  representations such as MPEG-4 (AAC), and similar to, but
 	  higher performance than MPEG-1/2 audio layer 3, MPEG-4 audio
 	  (TwinVQ), WMA and PAC.
+
+	  https://xiph.org/vorbis/