glx: meson: drop includes from a link-only library

When producing the final libGL.so/libGLX_mesa.so we only link the local
static helper lib (libglx). Thus there's no reason for the includes.

Fixes: a47c525f ("meson: build glx")
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
(cherry picked from commit b44875e2)