fix clippy warning

0.6.1 contains a clippy warning, which should taken care of

error: associated function `bootstrap_done` is never used
  --> crates/onionmasq-mobile/src/ffi_event.rs:82:12
   |
81 | impl TunnelEvent {
   | ---------------- associated function in this implementation
82 |     pub fn bootstrap_done() -> Self {
   |            ^^^^^^^^^^^^^^
   |