[gtkmm] gtk/src/expressionwatch.hg: Add #include <optional>

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 751444a4a3663520ad8406c229d4ba8af0f6b4f6
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Fri Jan 22 13:03:52 2021 +0100

    gtk/src/expressionwatch.hg: Add #include <optional>
    
    Fixes #84

 gtk/src/expressionwatch.hg | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/gtk/src/expressionwatch.hg b/gtk/src/expressionwatch.hg
index 78308d22..c727dd4b 100644
--- a/gtk/src/expressionwatch.hg
+++ b/gtk/src/expressionwatch.hg
@@ -18,6 +18,7 @@ _CONFIGINCLUDE(gtkmmconfig.h)
 
 #include <glibmm/object.h>
 #include <gtk/gtk.h>
+#include <optional>
 
 _DEFS(gtkmm,gtk)