license: Add OpenSSL exception to GPLv3 terms

We're using OpenSSL, so need to grant binary distrbutors permission to
link with it.

Cleared with current contributors.

Signed-off-by: Jeremy Kerr <jeremy.kerr@canonical.com>

image.h

@@ -15,6 +15,19 @@
  * along with this program; if not, write to the Free Software
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301,
  * USA.
+ *
+ * In addition, as a special exception, the copyright holders give
+ * permission to link the code of portions of this program with the OpenSSL
+ * library under certain conditions as described in each individual source file,
+ * and distribute linked combinations including the two.
+ *
+ * You must obey the GNU General Public License in all respects for all
+ * of the code used other than OpenSSL. If you modify file(s) with this
+ * exception, you may extend this exception to your version of the
+ * file(s), but you are not obligated to do so. If you do not wish to do
+ * so, delete this exception statement from your version. If you delete
+ * this exception statement from all source files in the program, then
+ * also delete it here.
  */
 #ifndef IMAGE_H
 #define IMAGE_H