Actually set the 'got_verstring' flag in sshshare.c!

For each connection to a downstream I had a flag indicating that we'd
sent a version string to that downstream, and one indicating that we'd
received one in return. But I never actually set the latter to TRUE -
which was OK, as it turned out, because I never used it for anything
either.

Now I do want to use it, so I'd better actually set it :-)

sshshare.c

@@ -1810,6 +1810,7 @@ static int share_receive(Plug plug, int urgent, char *data, int len)
     ssh_sharing_logf(cs->parent->ssh, cs->id,
                      "Downstream version string: %.*s",
                      cs->recvlen, cs->recvbuf);
+    cs->got_verstring = TRUE;
 
     /*
      * Loop round reading packets.