[libpng16] Removed "PNG_UNUSED()" inadvertently left in contrib/gregbook

/readpng2.c from the previous update.
diff --git a/contrib/gregbook/readpng2.c b/contrib/gregbook/readpng2.c
index 4150fa4..2e4c536 100644
--- a/contrib/gregbook/readpng2.c
+++ b/contrib/gregbook/readpng2.c
@@ -474,7 +474,6 @@
 {
     fprintf(stderr, "readpng2 libpng warning: %s\n", msg);
     fflush(stderr);
-    PNG_UNUSED(png_ptr)
     (void)png_ptr; /* Unused */
 }