[libhandy] swipeable: Add the missing <handy.h> inclusion guard

[Alexander Mikhaylenko <alexm src gnome org>]

commit 0e92772e36b37814f24bc4df7936d75f20c79640
Author: Adrien Plazas <kekun plazas laposte net>
Date:   Mon Jun 8 10:37:14 2020 +0200

    swipeable: Add the missing <handy.h> inclusion guard

 src/hdy-swipeable.h | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/src/hdy-swipeable.h b/src/hdy-swipeable.h
index fe0865ed..1212919d 100644
--- a/src/hdy-swipeable.h
+++ b/src/hdy-swipeable.h
@@ -6,6 +6,10 @@
 
 #pragma once
 
+#if !defined(_HANDY_INSIDE) && !defined(HANDY_COMPILATION)
+#error "Only <handy.h> can be included directly."
+#endif
+
 #include <gtk/gtk.h>
 #include "hdy-navigation-direction.h"