glx: Fix macOS build.

In file included from ../src/glx/apple/apple_glx_context.c:49: ../src/glx/glxclient.h:56:10: fatal error: 'loader.h' file not found ^~~~~~~~~~

Fixes: 1cb664c1 ("glx: s/dri_message/glx_message/") Closes: #4702 (closed) Signed-off-by: Vinson Lee vlee@freedesktop.org