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: 1cb664c15c ("glx: s/dri_message/glx_message/")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/4702
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10568>

src/glx/apple/apple_glx_log.c

@@ -35,6 +35,7 @@
 #include <stdlib.h>
 #include <inttypes.h>
 #include <pthread.h>
+#include "glxclient.h"
 #include "apple_glx_log.h"
 #include "util/debug.h"
 

src/glx/apple/apple_glx_pbuffer.c

@@ -44,6 +44,7 @@
 #include <stdlib.h>
 #include <pthread.h>
 #include <assert.h>
+#include "glxclient.h"
 #include "apple_glx.h"
 #include "glxconfig.h"
 #include "apple_cgl.h"

src/glx/apple/meson.build

@@ -54,7 +54,7 @@ endif
 libappleglx = static_library(
   'glxapple',
   [files_libappleglx, glapitable_h],
-  include_directories: [inc_mesa, inc_glx, inc_src, inc_include, inc_glapi],
+  include_directories: [inc_mesa, inc_glx, inc_src, inc_include, inc_glapi, inc_loader],
   dependencies: [dep_xext, dep_xplugin],
   gnu_symbol_visibility : 'hidden',
   build_by_default: false,