tcp: include netinet/tcp.h to make certain macros available

Following are tested: TCP_KEEPINTVL, TCP_KEEPIDLE, TCP_KEEPCNT

tcp_options.h

@@ -23,6 +23,7 @@
 
 #ifdef USE_TCP
 
+#include <netinet/tcp.h>
 #ifndef NO_TCP_ASYNC
 #define TCP_ASYNC /* enabled async mode */
 #endif