DO NOT MERGE Update libpng to 1.6.20

BUG:23265085
Change-Id: I85199805636d771f3597b691b63bc0bf46084833
(cherry picked from commit bbe98b40cda082024b669fa508931042eed18f82)
diff --git a/scripts/sym.c b/scripts/sym.c
new file mode 100644
index 0000000..ea9e4c5
--- /dev/null
+++ b/scripts/sym.c
@@ -0,0 +1,15 @@
+
+/* sym.c - define format of libpng.sym
+ *
+ * Last changed in libpng version 1.6.16 [December 22, 2014]
+ * Copyright (c) 2011-2014 Glenn Randers-Pehrson
+ *
+ * This code is released under the libpng license.
+ * For conditions of distribution and use, see the disclaimer
+ * and license in png.h
+ */
+
+#define PNG_EXPORTA(ordinal, type, name, args, attributes)\
+        PNG_DFN "@" SYMBOL_PREFIX "@@" name "@"
+
+#include "../png.h"