Convert OUString** to std::vector<OUString> in ScFuncDesc

There is no point in storing a ref-counted value type like OUString by
pointer.
And while we're there, convert array to std::vector

Change-Id: I5c0d3b7276e28956ec98852b52a6eaeb5a13dfed
Reviewed-on: https://gerrit.libreoffice.org/7057
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>