COMP: Fix compilation warnings with static char* reference strings

The warning:
vtkMRMLStorableNode.h:154: warning: the address of ‘vtkMRMLStorableNode::StorageNodeReferenceRole’ will always evaluate as ‘true’

When replacing char [] into char*, the new warning is:
/.../vtkMRMLStorableNode.cxx:27: warning: deprecated conversion from string constant to ‘char*’ [-Wwrite-strings]

When replacing char* by const char*, vtkGetStringMacro can't convert
from const char* to char*.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22214 3bd1e089-480b-0410-8dfb-8563597acbee