fdo#75757: remove inheritance to std::vector

Typedef'ing SvxXMLListStyle_Impl is enough.

Change-Id: I281766822aea60794510c959581f0b821aea6642
Reviewed-on: https://gerrit.libreoffice.org/10729
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>