glx: omit loader_loader() for macOS

glx: omit loader_loader() for macOS

Earlier commit added the code unconditionally, since the loader code
itself is already built on macOS.
Although it did not consider the #include mayhem that src/glx is.

In particular, none of the __GLXDRI{screen,context,drawable) are
available for macOS... those are pulled by dri_common.[ch].

Ideally we'll untangle that, but for the time being simply #ifdef out
the include/call.

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2726
Fixes: b699d070a6d ("glx: set the loader_logger early and for everyone")
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4490>