Fixed compile-time issue when symbol hiding is enabled (gcc 4.7)

From: Marco Clemencic <marco.clemencic@cern.ch>


git-svn-id: file:///afs/cern.ch/work/b/binet/dev/repos/gaudi-svn-mirror/Gaudi/trunk/GaudiKernel@7301 22b0f739-a4da-4a50-9948-337547c35a65