mtools: update upstream URL

The previously specified tux.org URL did not appear to be maintained.
The gnu.org URL looks better maintained and is the same domain as the
actual tarball download.

Signed-off-by: Frank Hunleth <fhunleth@troodon-software.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/mtools/Config.in.host

@@ -6,4 +6,4 @@ config BR2_PACKAGE_HOST_MTOOLS
 	  names, OS/2 Xdf disks and 2m disks (store up to 1992k on a high
 	  density 3 1/2 disk).
 
-	  http://www.tux.org/pub/tux/mtools/
+	  http://www.gnu.org/software/mtools/