Take into account wxFONTFLAG_STRIKETHROUGH in wxGTK wxFont ctor.

Honour wxFONTFLAG_STRIKETHROUGH in wxFont ctor taking flags as this is the
only way to create a strike-through font currently.

See #14559.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72487 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/font.h

@@ -318,6 +318,10 @@ protected:
         return (flags & wxFONTFLAG_UNDERLINED) != 0;
     }
 
+    static bool GetStrikethroughFromFlags(int flags)
+    {
+        return (flags & wxFONTFLAG_STRIKETHROUGH) != 0;
+    }
 
 private:
     // the currently default encoding: by default, it's the default system