Include GObject's visibility header

The gobject-introspection build goes through the GLib types when
generating the introspection data for GLib, but it does not include
glib-object.h, otherwise all GObject symbols would end up inside the
GLib namespace. This means we need to import the gobject-visibility.h
header inside glib-types.h. Since the header is guarded by a once
pragma, it doesn't really affect any legitimate user of the C API.

gobject/glib-types.h

@@ -24,6 +24,7 @@
 #endif
 
 #include <glib.h>
+#include <gobject/gobject-visibility.h>
 
 G_BEGIN_DECLS