[devel] Mention zlib-libpng license in each source file

pngget.c

diff --git a/pngget.c b/pngget.c
index 4d01c6e..632d325 100644
--- a/pngget.c
+++ b/pngget.c
@@ -2,10 +2,13 @@
 /* pngget.c - retrieval of values from info struct
  *
  * Last changed in libpng 1.4.0 [June 24, 2009]
- * For conditions of distribution and use, see copyright notice in png.h
  * Copyright (c) 1998-2009 Glenn Randers-Pehrson
  * (Version 0.96 Copyright (c) 1996, 1997 Andreas Dilger)
  * (Version 0.88 Copyright (c) 1995, 1996 Guy Eric Schalnat, Group 42, Inc.)
+ *
+ * This code is released under the zlib-libpng license.
+ * For conditions of distribution and use, see copyright notice in png.h
+ *
  */
 
 #include "png.h"