SessionIOJSON: add the create method

MSVC complains about it. It is used, so why GCC and Clang are happy is a
different story.

smtk/model/SessionIOJSON.h

@@ -26,6 +26,7 @@ class SMTKCORE_EXPORT SessionIOJSON : public SessionIO
 {
 public:
   smtkTypeMacro(SessionIOJSON);
+  smtkCreateMacro(SessionIOJSON);
 
   virtual int importJSON(ManagerPtr modelMgr, cJSON* sessionRec);
   virtual int exportJSON(ManagerPtr modelMgr, cJSON* sessionRec);