qt: add license header to source files

Closes #839

src/qt/bitcoinaddressvalidator.cpp

@@ -1,3 +1,7 @@
+// Copyright (c) 2011-2013 The Bitcoin developers
+// Distributed under the MIT/X11 software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
 #include "bitcoinaddressvalidator.h"
 
 /* Base58 characters are: