meson: set windows glx defines

Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>

meson.build

@@ -392,6 +392,8 @@ if with_platform_x11
     endif
     if with_dri_platform == 'drm'
       pre_args += '-DGLX_USE_DRM'
+    elif with_dri_platform == 'windows'
+      pre_args += '-DGLX_USE_WINDOWSGL'
     endif
   endif
 else