mission: fix warning about missing return

[julianoes]

@JonasVautherin is this the right fix, or should we add an enum named UNKNOWN?

It's a bit silly but it seems the compiler doesn't realize that all cases are covered and we don't need default.

The warning with gcc 8.1.1 is:

/home/julianoes/src/DroneCore/plugins/mission/mission_item.cpp: In function ‘std::ostream& dronecore::operator<<(std::ostream&, const dronecore::MissionItem::CameraAction&)’:
/home/julianoes/src/DroneCore/plugins/mission/mission_item.cpp:187:1: warning: control reaches end of non-void function [-Wreturn-type]
 }

[JonasVautherin]

Yep, seems like the compiler needs some help here :-).

LGTM