[My answer is wrong, but I've left it here because the comments are useful].
Forward declaring enums is non-standard, because pointers to different enum types are not guaranteed to be the same size. The compiler may need to see the definition to know what size pointers can be used with this type.
In practice, at least on all the popular compilers, pointers to enums are a consistent size. Forward declaration of enums is provided as a language extension by Visual C++, for example.