travis: adding missing x11-xcb for meson+vulkan

Required by the x11 WSI

Fixes: df82012b ("travis: add meson build for vulkan drivers.")
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
(cherry picked from commit 982e012b)