meson: fix wayland-less builds

meson: fix wayland-less builds

Those empty variables in the !wayland case are useless and running that
meson.build with them breaks the build:

  [287/850] Generating wayland-drm-client-protocol.h with a custom command.
  FAILED: src/egl/wayland/wayland-drm/wayland-drm-client-protocol.h
  client-header ../src/egl/wayland/wayland-drm/wayland-drm.xml src/egl/wayland/wayland-drm/wayland-drm-client-protocol.h
  /bin/sh: client-header: command not found
  ninja: build stopped: subcommand failed.

Fixes: d1992255bb29054fa5176 "meson: Add build Intel "anv" vulkan driver"
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>