sdl[2]_net: Fix winsocks2.h include

The problem didn't manifest itself until https://github.com/mirror/mingw-w64/commit/9bd8fe9148924840d315b4c915dd099955ea89d1
(included in MinGW-w64 3.3.0).

Fixes #550.

Signed-off-by: Timothy Gu <timothygu99@gmail.com>