Doxygen used to allow this but there was a regression, which is
apparently not being fixed:
https://github.com/doxygen/doxygen/issues/6304

Change-Id: I7602236522a56c30a2249bd8065796b3c6458704
Reviewed-on: https://gerrit.libreoffice.org/70377Reviewed-by: Michael Stahl <Michael.Stahl@cib.de>
Tested-by: Michael Stahl <Michael.Stahl@cib.de>