x11: only set up xfixes if it is actually present
x11: only set up xfixes if it is actually present

If an X server doesn't support the xfixes extension, we won't have it on
our connection any more, and we won't be able to set it up. Let's test for
this, and ignore the mypy errors since it doesn't understand hasattr() :)

Signed-off-by: Tycho Andersen <6b523322678240882acbb4da5312a12d8cd0082d@tycho.ws>

zordsdavini/qtile,zordsdavini/qtile