Fix unused_macros warning on ast_enum when features are disabled

    warning: unused macro definition: `ast_enum`
      --> src/macros.rs:40:14
       |
    40 | macro_rules! ast_enum {
       |              ^^^^^^^^
       |
       = note: `#[warn(unused_macros)]` on by default

src/macros.rs

@@ -37,6 +37,7 @@ macro_rules! ast_struct {
     };
 }
 
+#[cfg(any(feature = "full", features = "derive"))]
 macro_rules! ast_enum {
     (
         $(#[$enum_attr:meta])*