qmake locks moc files with RW lock on Windows, which causes spurious errors when qmake runs in parallel on profiles that share moc files

The error occurred because we run qmake in parallel in our build systems and some profiles share the same source files, so that
the same moc files get accessed by the qmakes.

It is correct to lock the file for writing, but it is wrong to lock it for reading,
the attached patch fixed this issue.