Make sure the library compiles for windows

systemd sockets are not applicable for windows. However compilation
should not fail on windows

fd_windows.go

@@ -0,0 +1,10 @@
+package anyhttp
+
+import (
+	"errors"
+	"net"
+)
+
+func makeFdListener(fd int, name string) (net.Listener, error) {
+	return nil, errors.New("windows not supported")
+}