engine: protect net engine IPv6 support with configure check

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/engines/net.c b/engines/net.c
index 4be106a..1dc55d5 100644
--- a/engines/net.c
+++ b/engines/net.c
@@ -94,18 +94,22 @@
 			    .oval = FIO_TYPE_TCP,
 			    .help = "Transmission Control Protocol",
 			  },
+#ifdef CONFIG_IPV6
 			  { .ival = "tcpv6",
 			    .oval = FIO_TYPE_TCP_V6,
 			    .help = "Transmission Control Protocol V6",
 			  },
+#endif
 			  { .ival = "udp",
 			    .oval = FIO_TYPE_UDP,
 			    .help = "User Datagram Protocol",
 			  },
+#ifdef CONFIG_IPV6
 			  { .ival = "udpv6",
 			    .oval = FIO_TYPE_UDP_V6,
 			    .help = "User Datagram Protocol V6",
 			  },
+#endif
 			  { .ival = "unix",
 			    .oval = FIO_TYPE_UNIX,
 			    .help = "UNIX domain socket",