CMake: Set warning level and disable -Waddress-of-packet-member (#7693)

* CMake: Set warning level and disable -Waddress-of-packet-member

Mavlink fails terribly on the check for -Waddress-of-packet-member
but this is not a problem of QGC, if I don't remove this warning
it's impossible to look at the terminal for QGC warnings.
This needs to be fixed however, but this is not the place.