syncqt: change to const-ref iteration variable in a for-loop

There is no need to copy the string from the container.

Change-Id: Idebe6a40e6f76ea22d8a8dc6d5d65f3fa277b9d9
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>

src/tools/syncqt/main.cpp

@@ -379,7 +379,7 @@ private:
             return false;
         }
 
-        for (auto header : generatedHeaders) {
+        for (const auto &header : generatedHeaders) {
             if (header.size() == 0)
                 continue;
             if (header[0] == '@') {