fdo#75757: remove inheritance to std::vector

from SfxFoundCacheArr_Impl, by typedef'ing it as
boost::ptr_vector<SfxFoundCache_Impl>.

Change-Id: Id0e50370b440fb53dbb56dabca9743b27a08b90d
Reviewed-on: https://gerrit.libreoffice.org/11650
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>