ipn/ipnserver: move BabysitProc to tailscaled_windows.go

It's only used by Windows. No need for it to be in ipn/ipnserver,
which we're trying to trim down.

Change-Id: Idf923ac8b6cdae8b5338ec26c16fb8b5ea548071
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f