mesa-gl: gallium is required when enabling x11

Otherwise Meson will fail wit this error:
  - meson.build:555:6: ERROR: Problem encountered: xlib based GLX requires at least one gallium driver

Related: https://github.com/agherzan/meta-raspberrypi/pull/1119


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>