[kernel] fix ast-diff on code with forward-decl of struct

In presence of the following pattern of code:

```c
struct S;
enum { t = 1 };
struct S { char[t]; };
```

-ast-diff will attempt to compare the use of `t` in the field declaration
before having visited the `enum` definition. Thus, we can't rely on an enumitem
being present in the correspondance table when looking for a correspondance.