Update to a hopefully more future proof FSF address

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/hw/xenfb.c b/hw/xenfb.c
index ad3e8ee..795a326 100644
--- a/hw/xenfb.c
+++ b/hw/xenfb.c
@@ -21,8 +21,7 @@
  *  GNU General Public License for more details.
  *
  *  You should have received a copy of the GNU General Public License along
- *  with this program; if not, write to the Free Software Foundation, Inc.,
- *  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ *  with this program; if not, see <http://www.gnu.org/licenses/>.
  */
 
 #include <stdarg.h>