meson: don't require dri2proto for darwin or windows

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>
diff --git a/meson.build b/meson.build
index 7a7793b..9222e6e 100644
--- a/meson.build
+++ b/meson.build
@@ -910,7 +910,9 @@
     endif
   endif
   if with_glx == 'dri'
-    dep_dri2proto = dependency('dri2proto', version : '>= 2.8')
+    if with_dri_platform == 'drm'
+      dep_dri2proto = dependency('dri2proto', version : '>= 2.8')
+    endif
     dep_glproto = dependency('glproto', version : '>= 1.4.14')
   endif
   if with_egl