libdpkg: Replace FSF address by pointing to the gnu.org URL

Use the <http://www.gnu.org/licenses/> URL, instead of the outdated FSF
address, which is way more stable, as the latter has changed several
times in the past.

lib/dpkg/t/t-tarextract.c

@@ -14,9 +14,8 @@
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU General Public License for more details.
  *
- * You should have received a copy of the GNU General Public
- * License along with dpkg; if not, write to the Free Software
- * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
 #include <config.h>