geant4: refind XercesC dependency

With the move to Geant4 static libraries, builds of Athena with Clang 14 exposed a linking error due to XercesC libraries appearing before the Geant4 archives (see athena!53390 (merged)). Traced to the FindGeant4 module not refinding the XercesC dependency and adding the libraries to the list of Geant4 libraries.

Refind XercesC in FindGeant4, adding the libraries to the list of libs in GEANT4_LIBRARIES. Tested in a full build with clang14 and allows the use of as-needed to be restored with clang14. That can addressed once this MR is merged and Athena uses the tag containing the fix.