gimbal: prevent segfault without gimbal v2 forced

It turns out this needed to be locked after all.

Signed-off-by: Julian Oes <julian@oes.ch>

src/mavsdk/plugins/gimbal/gimbal_impl.cpp

@@ -220,8 +220,7 @@ Gimbal::ControlHandle GimbalImpl::subscribe_control(const Gimbal::ControlCallbac
 
  if (result.first) {
  wait_for_protocol_async([=]() {
- // We don't lock here because we don't expect the protocol to change once it has been
- // set.
+ std::lock_guard<std::mutex> lock(_mutex);
  _gimbal_protocol->control_async([this](Gimbal::ControlStatus status) {
  _control_subscriptions.queue(
  status, [this](const auto& func) { _system_impl->call_user_callback(func); });
@@ -254,8 +253,7 @@ Gimbal::AttitudeHandle GimbalImpl::subscribe_attitude(const Gimbal::AttitudeCall
 
  if (result.first) {
  wait_for_protocol_async([=]() {
- // We don't lock here because we don't expect the protocol to change once it has been
- // set.
+ std::lock_guard<std::mutex> lock(_mutex);
  _gimbal_protocol->attitude_async([this](Gimbal::Attitude attitude) {
  _attitude_subscriptions.queue(
  attitude, [this](const auto& func) { _system_impl->call_user_callback(func); });