Do not use C++11 std::vector.data()

std::vector.data() is a C++11 feature that makes my OSX build machine unhappy.

src/rpcmining.cpp

@@ -307,7 +307,7 @@ Value getblocktemplate(const Array& params, bool fHelp)
         }
         entry.push_back(Pair("depends", deps));
 
-        int index_in_template = &tx - pblock->vtx.data();
+        int index_in_template = i - 1;
         entry.push_back(Pair("fee", pblocktemplate->vTxFees[index_in_template]));
         entry.push_back(Pair("sigops", pblocktemplate->vTxSigOps[index_in_template]));