[gtk+] CenterBox: Guard against directly including header

[Daniel Boles <dboles src gnome org>]

commit 4accb9f1679f08ab64a945800d733f51361d0f42
Author: Daniel Boles <dboles src gnome org>
Date:   Sat Aug 5 18:44:02 2017 +0100

    CenterBox: Guard against directly including header
    
    https://bugzilla.gnome.org/show_bug.cgi?id=785280

 gtk/gtkcenterbox.h |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)
---
diff --git a/gtk/gtkcenterbox.h b/gtk/gtkcenterbox.h
index 4c8e7ad..bfd68f8 100644
--- a/gtk/gtkcenterbox.h
+++ b/gtk/gtkcenterbox.h
@@ -22,6 +22,10 @@
 #ifndef __GTK_CENTER_BOX_H__
 #define __GTK_CENTER_BOX_H__
 
+#if !defined (__GTK_H_INSIDE__) && !defined (GTK_COMPILATION)
+#error "Only <gtk/gtk.h> can be included directly."
+#endif
+
 #include "gtkwidget.h"
 
 G_BEGIN_DECLS