ident: fix undefined variable when NO_IPV6 is set

Commit 00bce77 (ident.c: add support for IPv6, 2015-11-27)
moved the "gethostbyname" call out of "add_domainname" and
into the helper function "canonical_name". But when moving
the code, it forgot that the "buf" variable is passed as
"host" in the helper.

Reported-by: johan defries <johandefries@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ident.c

@@ -86,7 +86,7 @@ static int canonical_name(const char *host, struct strbuf *out)
 		freeaddrinfo(ai);
 	}
 #else
-	struct hostent *he = gethostbyname(buf);
+	struct hostent *he = gethostbyname(host);
 	if (he && strchr(he->h_name, '.')) {
 		strbuf_addstr(out, he->h_name);
 		status = 0;