Add netinet/tcp.h to includes to support TCP socket options on *nix.

src/structures.h

@@ -28,6 +28,7 @@ extern "C" {
 #ifndef _WIN32
 #include <sys/socket.h>
 #include <netinet/in.h>
+#include <netinet/tcp.h>
 #include <arpa/inet.h>
 #include <netdb.h>
 #include <pthread.h>