rwcancel: no-op builds for windows and darwin

This lets us include the package on those platforms in a
followup commit where we split out a conn package from device.
It also lets us run `go test ./...` when developing on macOS.

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>

rwcancel/rwcancel_windows.go

@@ -0,0 +1,8 @@
+// SPDX-License-Identifier: MIT
+
+package rwcancel
+
+type RWCancel struct {
+}
+
+func (*RWCancel) Cancel() {}