DO NOT MERGE Update libpng to 1.6.20

BUG:23265085
Change-Id: I85199805636d771f3597b691b63bc0bf46084833
(cherry picked from commit bbe98b40cda082024b669fa508931042eed18f82)

void readpng_version_info(void)
{
    fprintf(stderr, "   Compiled with libpng %s; using libpng %s.\n",
      PNG_LIBPNG_VER_STRING, png_libpng_ver);
    fprintf(stderr, "   Compiled with zlib %s; using zlib %s.\n",
      ZLIB_VERSION, zlib_version);
}

void readpng_version_info(void)
{
    fprintf(stderr, "   Compiled with libpng %s; using libpng %s.\n",
      PNG_LIBPNG_VER_STRING, png_libpng_ver);
    fprintf(stderr, "   Compiled with zlib %s; using zlib %s.\n",
      ZLIB_VERSION, zlib_version);
}
