MINOR: quic: Wrong packet flags settings during frame building

We flag the packet as being ack-eliciting when building the frame.
But a wrong variable was used to to so.

src/quic_frame.c

@@ -1016,7 +1016,7 @@ int qc_build_frm(unsigned char **buf, const unsigned char *end,
 		return 0;
 	}
 
-	pkt->flags |= builder[frm->type].flags;
+	pkt->flags |= builder->flags;
 
 	return 1;
 }