fix issue of winsocks not being initialized

[hlimbo]

Fixes the following issue when you try to run ggpo separately from vectorwars (e.g. building your own ggpo.lib and ggpo.dll files):

Proposed Fix

• Before ggpo_start_session() function is called make sure to call gpo_initialize_winsock() first.

[hlimbo]

For more context see: https://github.com/pond3r/ggpo/issues/68