Pulls in a patch I've upstreamed to miniupnpc so that we properly pass our cflags when building it for mingw. See miniupnp/miniupnp#619. Also set D_WIN32_WINNT to 0x0601 to match libevent, configure etc. Previously it was being set to 0X501.
Backport of core#25964.
Depends on D16562.