We can't infer the right type for voidp for old png_jmpbuf()

In libpng 1.2 it's just a macro that's ->jmpbuf,
so there's nothing forcing the conversion to png_structp.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2268953005

Review-Url: https://codereview.chromium.org/2268953005
1 file changed