[FTBFS w/-unity-build] Linux build runs into sys/mount.h vs. linux/fs.h incompatibility

https://codereview.qt-project.org/c/qt/qtbase/+/511801 broke -unity-build's in which qfilesystemengine_unix.cpp ends up in the same unity_cxx_nn.cpp as qstorageinfo_linux.cpp.

While this affects only certain glibc's, Unbuntu 20.04 is affected. The internet suggests to not mix the two APIs in one executable, due to ODR violations (MS_ flags are #define'd in one and an enum in the other).

Reference:

• https://sourceware.org/glibc/wiki/Release/2.36#Usage_of_.3Clinux.2Fmount.h.3E_and_.3Csys.2Fmount.h.3E