Add a blank 3rd clause to the otherwise 2-clause BSD

Debian packaging people did not like the discrepancy between the label BSD3
and the content of the LICENSE file which ommits the standard 3rd clause.

LICENSE

@@ -9,6 +9,7 @@ modification, are permitted provided that the following conditions are met:
 2. Redistributions in binary form must reproduce the above copyright
    notice, this list of conditions and the following disclaimer in the
    documentation and/or other materials provided with the distribution.
+3. This clause is intentionally left blank.
 
 THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE