add DEFAULT_TRANSACTION_FEE constant in wallet

- as this is a shared Core/GUI setting, this makes it easier to keep them
  in sync (also no new includes are needed)

src/wallet.cpp

@@ -6,9 +6,9 @@
 #include "wallet.h"
 
 #include "base58.h"
+#include "checkpoints.h"
 #include "coincontrol.h"
 #include "net.h"
-#include "checkpoints.h"
 
 #include <boost/algorithm/string/replace.hpp>
 #include <openssl/rand.h>
@@ -16,7 +16,7 @@
 using namespace std;
 
 // Settings
-int64_t nTransactionFee = 0;
+int64_t nTransactionFee = DEFAULT_TRANSACTION_FEE;
 bool bSpendZeroConfChange = true;
 
 //////////////////////////////////////////////////////////////////////////////