component/qt5-qmake: Fix build failure on systems with libxcb
Hello,
recently building Cloudooo failed for me on a machine with Ubuntu 20.04.6 LTS. On a very similar machine with the same OS, it worked okay. From the build error (see commit message) this looks very much like an accidental dependency on system packages. Adding the flag to use the qt included version of libxcb seems to fix the problem for me.
Let's see if tests run ok.
Best, Levin