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

pngtest.c

diff --git a/pngtest.c b/pngtest.c
index 279279c..8f5c1b0 100644
--- a/pngtest.c
+++ b/pngtest.c
@@ -2,11 +2,13 @@
 /* pngtest.c - a simple test program to test libpng
  *
  * 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
+ *
  * This program reads in a PNG image, writes it out again, and then
  * compares the two files.  If the files are identical, this shows that
  * the basic chunk handling, filtering, and (de)compression code is working