Make sure the library compiles for windows

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

fd_unix.go

@@ -0,0 +1,19 @@
+//go:build unix
+
+package anyhttp
+
+import (
+	"net"
+	"os"
+	"syscall"
+)
+
+func makeFdListener(fd int, name string) (net.Listener, error) {
+	fdFile := os.NewFile(uintptr(fd), name)
+	l, err := net.FileListener(fdFile)
+	if err != nil {
+		return nil, err
+	}
+	syscall.CloseOnExec(fd)
+	return l, nil
+}