Core (LV::Singleton): Initialize the static instance pointers using 'inline'.

Author: kaixiong

libvisual/libvisual/lv_singleton.hpp

@@ -39,14 +39,11 @@ namespace LV {
 
   protected:
 
-      static std::unique_ptr<T> m_instance;
+      inline static std::unique_ptr<T> m_instance {};
 
       Singleton () = default;
   };
 
-  template <class T>
-  std::unique_ptr<T> Singleton<T>::m_instance {};
-
 } // LV namespace
 
 #endif // _LV_SINGLETON_HPP