Add the <arpa/inet.h> include to unix.h.

Some files were using ENET_HOST_TO_NET_32 and ENET_NET_TO_HOST_32 without having included the <arpa/inet.h> file beforehand, leading to compiler warnings about implicit declarations of ntohl and htonl.

include/enet/unix.h

@@ -9,6 +9,7 @@
 #include <sys/time.h>
 #include <sys/types.h>
 #include <sys/socket.h>
+#include <arpa/inet.h>
 #include <netinet/in.h>
 #include <unistd.h>