Check for IPV6_TCLASS instead of IPPROTO_IPV6 since
it's present on non-IPv6 enabled toolchains too.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
---

diff -Nura dropbear-2011.54.orig/dbutil.c dropbear-2011.54/dbutil.c
--- dropbear-2011.54.orig/dbutil.c	2011-11-08 09:48:15.000000000 -0300
+++ dropbear-2011.54/dbutil.c	2011-11-09 12:14:59.430074138 -0300
@@ -164,7 +164,7 @@
 	/* set the TOS bit for either ipv4 or ipv6 */
 #ifdef IPTOS_LOWDELAY
 	val = IPTOS_LOWDELAY;
-#ifdef IPPROTO_IPV6
+#ifdef IPV6_TCLASS
 	setsockopt(sock, IPPROTO_IPV6, IPV6_TCLASS, (void*)&val, sizeof(val));
 #endif
 	setsockopt(sock, IPPROTO_IP, IP_TOS, (void*)&val, sizeof(val));