embed macOS const to avoid platform inconsistency(again)

transport/internet/sockopt_darwin.go

@@ -28,7 +28,7 @@ func applyOutboundSocketOptions(network string, address string, fd uintptr, conf
 		}
 
 		if config.TcpKeepAliveInterval > 0 {
-			if err := syscall.SetsockoptInt(int(fd), syscall.IPPROTO_TCP, syscall.TCP_KEEPINTVL, int(config.TcpKeepAliveInterval)); err != nil {
+			if err := syscall.SetsockoptInt(int(fd), syscall.IPPROTO_TCP, TCP_KEEPINTVL, int(config.TcpKeepAliveInterval)); err != nil {
 				return newError("failed to set TCP_KEEPINTVL", err)
 			}
 			if err := syscall.SetsockoptInt(int(fd), syscall.SOL_SOCKET, syscall.SO_KEEPALIVE, 1); err != nil {