contrib: png: Don't require runtime checks for NEON on arm on win32

NEON is always available there, just like on iOS.

This fixes building for Windows on ARM after switching to building libpng with CMake - after that change, building failed due to "No support for run-time ARM Neon checking; use compile-time options".

(In theory, this could also be fixed upstream in various ways - since the compiler defines __ARM_NEON, it is clear that NEON is universally usable without runtime checks for this target. However, I tried to upstream a fix for that, and it turned into a big mess; https://sourceforge.net/p/png-mng/mailman/png-mng-implement/thread/CAP7U39_TrtYk0EXk7-XYMSnGO36afzpz5BNvNPETJvb-zj0xCQ%40mail.gmail.com/ The main consensus there at least, is that the cmake build of png is worse and more incorrect than the configure build, and the configure script is the canonical way of doing it, where the user has to explicitly opt in to NEON opts.)