[libpng16] Added "(void) png_ptr;" where needed in contrib/gregbook to quiet

compiler complaints about unused pointers.
diff --git a/CHANGES b/CHANGES
index 3d169d9..fac56b5 100644
--- a/CHANGES
+++ b/CHANGES
@@ -4943,6 +4943,8 @@
 Version 1.6.13beta01 [June 13, 2014]
   Quieted -Wsign-compare and -Wclobber compiler warnings in
     contrib/pngminus/*.c
+  Added "(void) png_ptr;" where needed in contrib/gregbook to quiet
+    compiler complaints about unused pointers.
 
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