added missing template arguments to std::unique_ptr declaration

Function.hpp

@@ -32,7 +32,7 @@ private:
         virtual ~callable() override {}
     };
 
-    std::unique_ptr _fun;
+    std::unique_ptr<callable_base> _fun;
 
 public:
     Function() {}
@@ -49,4 +49,4 @@ public:
 
     operator bool() const
     { return static_cast<bool>(_fun); }
-};
+};