[libpng16] Update "last changed" date in pngtrans.c

pngtrans.c

 
 /* pngtrans.c - transforms the data in a row (used by both readers and writers)
  *
- * Last changed in libpng 1.6.30 [June 28, 2017]
+ * Last changed in libpng 1.6.33 [(PENDING RELEASE)]
  * Copyright (c) 1998-2002,2004,2006-2017 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.)