QtWayland links against Xext (but it shouldn't)

Since Qt 5.6 Xext is needed to build QtWayland module, otherwise build fails while linking libraries, this dependency was not required with Qt 5.5 and we would like to avoid that since it makes harder to have a X11-free wayland based system.

ldd libQt5Compositor.so.5 | grep Xext
libXext.so.6 => /usr/lib/x86_64-linux-gnu/libXext.so.6 (0x00007fdbfd1a8000)