Update LGPL-2 license grant.

Avoid outdated FSF postal address.

wrlib/gradient.c

@@ -16,9 +16,8 @@
  *  Library General Public License for more details.
  *
  *  You should have received a copy of the GNU Library General Public
- *  License along with this library; if not, write to the Free
- *  Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston,
- *  MA 02110-1301, USA.
+ *  License along with this library; if not, see
+ *  <https://www.gnu.org/licenses/>.
  */
 
 #include <config.h>