Add missing "override" keyword
Add missing "override" keyword

This fixes a warning in at least one (clang + InfFE + paranoid warnings)
configuration.

dschwen/libmesh,dschwen/libmesh,libMesh/libmesh,libMesh/libmesh,jwpeterson/libmesh,dschwen/libmesh,dschwen/libmesh,dschwen/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,libMesh/libmesh,libMesh/libmesh,dschwen/libmesh,dschwen/libmesh,libMesh/libmesh,libMesh/libmesh,jwpeterson/libmesh,dschwen/libmesh,libMesh/libmesh,jwpeterson/libmesh,libMesh/libmesh,jwpeterson/libmesh