[gtk/wip/jimmac/global-var] Adwaita: make parser happy wrt global variables

[Jakub Steiner <jimmac src gnome org>]

commit 926764be0dc248de29d3026f807ecfcf1c2af6b0
Author: Jakub Steiner <jimmac gmail com>
Date:   Wed May 6 14:12:44 2020 +0200

    Adwaita: make parser happy wrt global variables
    
    Fixes https://gitlab.gnome.org/GNOME/gtk/-/issues/2703

 gtk/theme/Adwaita/_common.scss | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/gtk/theme/Adwaita/_common.scss b/gtk/theme/Adwaita/_common.scss
index 7cd37b67e6..5e8342f864 100644
--- a/gtk/theme/Adwaita/_common.scss
+++ b/gtk/theme/Adwaita/_common.scss
@@ -11,6 +11,7 @@ $button_radius: 5px;
 $menu_radius: 5px;
 $window_radius: $button_radius + 3;
 $popover_radius: $button_radius + 4;
+$button_fill: null; // global var defined within a parameter driven mixin. See issue #2703
 
 /***************
  * Base States *