expose SaveToFile symbol on windows (#1685)

include/tvm/runtime/module.h

@@ -103,8 +103,8 @@ class ModuleNode {
    * \param file_name The file to be saved to.
    * \param format The format of the file.
    */
-  virtual void SaveToFile(const std::string& file_name,
-                          const std::string& format);
+  TVM_DLL virtual void SaveToFile(const std::string& file_name,
+                                  const std::string& format);
   /*!
    * \brief Save the module to binary stream.
    * \param stream The binary stream to save to.