ftp: don't build ftp.c for non-6711 hw

modules/ftp.c

@@ -1,3 +1,4 @@
+#ifdef HARDWARE_BT6711
 /*
  * lwftp.c : a lightweight FTP client using raw API of LWIP
  *
@@ -682,3 +683,5 @@ void start_ftp_client(lwftp_session_t *s)
   lwftp_retr(s);
   // FTP session will continue with the connection callback
 }
+
+#endif // HARDWARE_BT6711