haiku: Switch from e! to c_enum!

The enums used on Haiku are currently broken: they have no `repr`
attribute so Rust is treating them as a `u8` when they need to be
`c_int`. This is flagged by `improper_ctypes`, which only shows up now
that this lint got removed.

Since this is broken anyway, we may as well just switch to `c_enum!`.
This is a user-visible breaking change, but allowed as the target is
tier 3.

Partially cherry picked from commit a541bf4 ("libc: remove uses of
enum as per #4419").

Co-authored-by: mbyx <[email protected]>

Author: tgross35

src/unix/haiku/native.rs

@@ -46,7 +46,7 @@ pub type thread_func = extern "C" fn(*mut c_void) -> status_t;
 // kernel/image.h
 pub type image_id = i32;
 
-e! {
+c_enum! {
     // kernel/OS.h
     pub enum thread_state {
         B_THREAD_RUNNING = 1,