Fix invalid + of std::string and int

These warnings were emitted by clang++, and they are real bugs.
src/rpc/core_rpc_server.cpp:208:58: warning: adding 'uint64_t'
      (aka 'unsigned long') to a string does not append to the string
      [-Wstring-plus-int]
        res.status = "Error retrieving block at height " + height;
                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~
The obvious intent is achieved by using std::to_string().

src/rpc/core_rpc_server.cpp

@@ -205,7 +205,7 @@ namespace cryptonote
       }
       catch (...)
       {
-        res.status = "Error retrieving block at height " + height;
+        res.status = "Error retrieving block at height " + std::to_string(height);
         return true;
       }
       std::list<transaction> txs;