[gtksourceview] preview: add missing G_BEGIN_DECLS

[Christian Hergert <chergert src gnome org>]

commit 04337bf51870e4deb3538fab2924b5825f8bc312
Author: Percy Zahl <pyzahl gmail com>
Date:   Mon Jan 31 15:10:56 2022 +0000

    preview: add missing G_BEGIN_DECLS

 gtksourceview/gtksourcestyleschemepreview.h | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/gtksourceview/gtksourcestyleschemepreview.h b/gtksourceview/gtksourcestyleschemepreview.h
index 0ef7889f..8cdbf727 100644
--- a/gtksourceview/gtksourcestyleschemepreview.h
+++ b/gtksourceview/gtksourcestyleschemepreview.h
@@ -29,6 +29,8 @@
 
 #include "gtksourcetypes.h"
 
+G_BEGIN_DECLS
+
 #define GTK_SOURCE_TYPE_STYLE_SCHEME_PREVIEW (gtk_source_style_scheme_preview_get_type())
 
 GTK_SOURCE_AVAILABLE_IN_5_4