Fix writeXMLHeader overloading with multi-inheritance

The previous patch only partially resolved the issue. We have
multi-inheritance here and both base classes contain` writeXMLHeader`, yet
both have different signatures.

This patch resolves all warnings related to `writeXMLHeader` and should 
resolve the last diagnostic error on Darwin in Boost Serialization.

Compile tested on GCC and Clang.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>