message: add missing header ifdef
diff: // Distributed under the MIT software license, see the accompanying
  // file COPYING or http://www.opensource.org/licenses/mit-license.php.
  +#ifndef BITCOIN_TXADMISSION_H
  +#define BITCOIN_TXADMISSION_H
  +
  #include "fastfilter.h"
  #include "net.h"
  #include "threadgroup.h"
  @@ -217,3 +220,5 @@ public:
  txProcessingCorral.Exit(CORRAL_TX_PAUSE);
  }
  };
  +
  +#endif
  