Fix build with gcc-14.

Backported version of patch from:
https://gitlab.gnome.org/GNOME/gtksourceview/-/commit/b25e71c57fc934a7ce36e51826af9fa7c2cf9a80

Bug: https://bugs.gentoo.org/937626

Signed-off-by: Holger Hoffstätte <holger@applied-asynchrony.com>

--- gtksourceview-2.10.5/gtksourceview/gtksourceview.c	2010-08-22 13:45:05.000000000 +0200
+++ gtksourceview-2.10.5-gcc14/gtksourceview/gtksourceview.c	2024-08-09 16:11:37.319493797 +0200
@@ -1766,7 +1766,7 @@ set_source_buffer (GtkSourceView *view,
 
 	if (buffer && GTK_IS_SOURCE_BUFFER (buffer))
 	{
-		view->priv->source_buffer = g_object_ref (buffer);
+		view->priv->source_buffer = g_object_ref (GTK_SOURCE_BUFFER (buffer));
 
 		g_signal_connect (buffer,
 				  "highlight_updated",