[gtk/gtk-3-24] Add a missing include

[Matthias Clasen <matthiasc src gnome org>]

commit b1d3beedc3cbabc5050a4292ec398978ca873071
Author: Matthias Clasen <mclasen redhat com>
Date:   Wed Sep 12 23:02:26 2018 -0400

    Add a missing include
    
    Closes https://gitlab.gnome.org/GNOME/gtk/issues/1325

 gtk/gtkprintoperation-portal.c | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/gtk/gtkprintoperation-portal.c b/gtk/gtkprintoperation-portal.c
index fb3dad0f04..9fb0226706 100644
--- a/gtk/gtkprintoperation-portal.c
+++ b/gtk/gtkprintoperation-portal.c
@@ -37,6 +37,7 @@
 #include "gtkshow.h"
 #include "gtkintl.h"
 #include "gtkwindowprivate.h"
+#include "gtkprivate.h"
 
 
 typedef struct {