w3m-nox: fix build

In the latest version (0.5.3+git20190105), `<X11/xlib.h>` isn't directly
included in `fb_imlib2.c` anymore, however it's used by `<Imlib.h>`
which broke our patch which disables X11 here.

Hydra build failure: https://hydra.nixos.org/build/93730733
See also https://github.com/tats/w3m/commit/2266d1bd988daed273992e7b26fbeba030002b82