Add one more missing dep_x11_headers

I missed this in 396c10bd since I was
building with -Dglx=disabled. This fixes the build with glx if the X11
libraries are installed into a custom prefix.

Signed-off-by: Alex Richardson <Alexander.Richardson@cl.cam.ac.uk>
12 jobs for one-more-x11-dep in 1 minute and 22 seconds (queued for 12 seconds)