Disabling warnings about missing return

GCC's C++ compiler may complain about that the function

OnlineSearchGeneral::Form::Private::queryKeyToString

does reach its end without returning anything. Seemingly,
the compiler is not aware that the switch covers all cases
of the OnlineSearchAbstract::QueryKey enum.
Unfortunately, this problem causes the C++ compiler inside
GitLab's CI to stop and issue an error, making the CI to
abort.
Thus, this particular check gets temporarily disabled.