Revert "Temporarily remove "final" from chart::FormattedString"

This reverts commit f43b5d1e, now that the
experiment is done.

Change-Id: I0f483023c7f9f4f5ae1edaa7abb7056e1c4e30a1
Reviewed-on: https://gerrit.libreoffice.org/72129
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

chart2/source/model/main/FormattedString.hxx

@@ -43,7 +43,7 @@ typedef ::cppu::WeakImplHelper<
     FormattedString_Base;
 }
 
-class FormattedString :
+class FormattedString final :
     public MutexContainer,
     public impl::FormattedString_Base,
     public ::property::OPropertySet