#include netinet/tcp.h so TCP_* will actually be defined

This header needs to be included so the TCP_* defines are available.
Sockets created with open-client-stream and open-server-stream would
try to set TCP_NODELAY if available, but they couldn't because this
was not defined.

Tested on OpenBSD and Linux.

src/c/tcp.d

@@ -28,6 +28,7 @@ extern int errno;
 #include <sys/socket.h>
 #include <sys/un.h>
 #include <netinet/in.h>
+#include <netinet/tcp.h>
 #include <arpa/inet.h>
 #include <netdb.h>
 #include <unistd.h>