The Makefile unconditionally linked libX11-xcb into libvulkan_intel.so.
But it's needed only if HAVE_PLATFORM_X11.
Fixes build of libvulkan_intel.so on Chromium OS, which has no X11
libraries.
Fixes: 71258e9462 ("anv/x11: Add support for Xlib platform")
Cc: Kevin Strasser <kevin.strasser@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>