linknx: add additional upstream URL

While the SourceForge page still exists, it seems to be a bit outdated,
and there is now a much newer Github repository with recent activity, so
mention it as well in the Config.in help text.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/linknx/Config.in

@@ -11,6 +11,7 @@ config BR2_PACKAGE_LINKNX
 	  to EIB/KNX installation.
 
 	  http://linknx.sourceforge.net/
+	  https://github.com/linknx/linknx
 
 comment "linknx needs a toolchain w/ C++"
 	depends on BR2_USE_MMU