QDoc fails to parse Qt headers when extra includepaths are set in the qdocconf

When the qdocconf contains an extra includepaths the command-line args passed to clang are broken and by that parsing Qt headers doesn't work, which in the end means that no C++ classes can be documented.

I stumbled over this when working on https://codereview.qt-project.org/#/c/256891/

It can be reproduced by adding the following line to the qddocconf:

includepaths += $$BUILDDIR/../src/remotesettings/frontend