package/gtkperf: 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: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/gtkperf/Config.in

@@ -7,3 +7,5 @@ config BR2_PACKAGE_GTKPERF
 	  predefined GTK+ widgets
 	  e.g. (opening comboboxes, toggling buttons, scrolling text
 	  yms.) and this way define the speed of device/platform.
+
+	  http://gtkperf.sourceforge.net/