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

compiler complaints about unused pointers.

contrib/gregbook/readpng2.c

@@ -448,6 +448,8 @@ static void readpng2_end_callback(png_structp png_ptr, png_infop info_ptr)
 
     /* all done */
 
+    (void)info_ptr; /* Unused */
+
     return;
 }
 
@@ -472,6 +474,8 @@ static void readpng2_warning_handler(png_structp png_ptr, png_const_charp msg)
 {
     fprintf(stderr, "readpng2 libpng warning: %s\n", msg);
     fflush(stderr);
+    PNG_UNUSED(png_ptr)
+    (void)png_ptr; /* Unused */
 }