nfs-utils: fix build with ipv6 after 2a10c9 (pass --enable-ipv6 to autotargets configure)

IPv6 support needs the TIRPC library, which we don't have in Buildroot, so
forcibly disable IPv6 support.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/nfs-utils/nfs-utils.mk

@@ -14,6 +14,7 @@ NFS_UTILS_CONF_OPT = \
 		--disable-gss \
 		--disable-tirpc \
 		--disable-uuid \
+		--disable-ipv6 \
 		--without-tcp-wrappers \
 		--with-rpcgen=internal