Fix/windows port finishing

[SCLevi]

[raggi]

When I looked at this previously, I found that Go's support for buildmode=c-shared wasn't implemented for windows. How are you building the shared library?

[balazsbencs-sc]

Let me answer instead of Levi, since we've worked together on this a little bit: We are using TDM GCC on windows to build this.

$env:CGO_ENABLED = '1'
$env:CGO_LDFLAGS = '-lws2_32'
$env:CC = 'gcc'
go build -buildmode=c-shared -o libtailscale.dll -x