netlabel: Kconfig: Update reference for NetLabel Tools project

The NetLabel Tools project has moved from http://netlabel.sf.net

 to a
GitHub project. Update to directly refer to the new home for the tools.

Signed-off-by: Salvatore Bonaccorso <carnil@debian.org>
Acked-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: David S. Miller <davem@davemloft.net>