[MOD] Use server_base instead of the API host

This wasn't an issue because the server I test on has the same API host
as its name.

src/XMPPThread/Bridged.c

@@ -184,7 +184,7 @@ ScrambleOID(ParseeData *data, char *opaque_oid)
     mxid = StrConcat(
         6,
         "@", c->namespace_base, "_l_", sha, ":",
-        c->homeserver_host
+        c->server_base
     );
     Free(sha);
     return mxid;