third_party/libpng: update to 1.2.54

TBR=darin@chromium.org
BUG=560291

Review URL: https://codereview.chromium.org/1467263003

Cr-Commit-Position: refs/heads/master@{#362298}
png_mmx_support(void)
{
   /* Obsolete, to be removed from libpng-1.4.0 */
    return -1;
}

png_mmx_support(void)
{
   /* Obsolete, to be removed from libpng-1.4.0 */
    return -1;
}
