test719: require ipv6 support to run

Follow-up to effd2bd7ba
Reported-by: Marc Hörsken
Bug: https://github.com/curl/curl/pull/8217#issuecomment-1004681145

Closes #8223

tests/data/test719

@@ -30,6 +30,9 @@ Funny-head: yesyes
 #
 # Client-side
 <client>
+<features>
+ipv6
+</features>
 <server>
 http
 socks5