cmServerProtocol: Do not move into json::Value::append()

CMake's copy of json-cpp is not aware of rvalues yet.  It is confusing
to pass the result of std::move to a function that takes a const&
because no move will actually happen.  This change may be reverted once
CMake upgrades to a new version of json-cpp.

Source/cmServerProtocol.cxx

@@ -747,7 +747,7 @@ static Json::Value DumpBacktrace(const cmListFileBacktrace& backtrace)
     if (!backtraceCopy.Top().Name.empty()) {
       entry[kNAME_KEY] = backtraceCopy.Top().Name;
     }
-    result.append(std::move(entry));
+    result.append(entry);
     backtraceCopy = backtraceCopy.Pop();
   }
   return result;