Build fix.

osdep/LinuxEthernetTap.cpp

@@ -440,7 +440,7 @@ void LinuxEthernetTap::threadMain()
 			return;
 		}
 
-		ifr.ifr_ifru.ifru_mtu = (int)mtu;
+		ifr.ifr_ifru.ifru_mtu = (int)_mtu;
 		if (ioctl(sock,SIOCSIFMTU,(void *)&ifr) < 0) {
 			::close(sock);
 			printf("WARNING: ioctl() failed setting up Linux tap device (set MTU)\n");