prepend license statement to indirectmap

Add statement about MIT licensing to indirectmap.h. I forgot the license
preamble when I originally wrote the file.

src/indirectmap.h

@@ -1,3 +1,7 @@
+// Copyright (c) 2016 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
 #ifndef BITCOIN_INDIRECTMAP_H
 #define BITCOIN_INDIRECTMAP_H