Make attribute::get_atoms a virtual function override

include/c74_min_attribute.h

@@ -159,6 +159,7 @@ class attribute_base
     // All attributes must define what happens when you get their value.
 
     virtual operator atoms() const = 0;
+    virtual atoms get_atoms() const = 0;
 
     // All attributes must define what happens when asked for their range of values.
     // The range must be in string format, values separated by spaces.
@@ -649,7 +650,7 @@ class attribute : public attribute_base
     /// Get the attribute value as a vector of atoms.
     /// @return The value of the attribute
 
-    atoms get_atoms() const
+    atoms get_atoms() const override
     {
         if (m_getter) {
             return m_getter();