[retro-gtk] retro-glsl-filter: Add license header

[Adrien Plazas <aplazas src gnome org>]

commit f6c2548f79aaa7da0f0dce3f827391fae6665c01
Author: Alexander Mikhaylenko <exalm7659 gmail com>
Date:   Sun Sep 1 20:50:45 2019 +0500

    retro-glsl-filter: Add license header

 retro-gtk/retro-glsl-filter.h | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/retro-gtk/retro-glsl-filter.h b/retro-gtk/retro-glsl-filter.h
index 2b451ef..63d7758 100644
--- a/retro-gtk/retro-glsl-filter.h
+++ b/retro-gtk/retro-glsl-filter.h
@@ -1,3 +1,5 @@
+// This file is part of retro-gtk. License: GPL-3.0+.
+
 #pragma once
 
 #include <epoxy/gl.h>