rust/app-layer: expose AppLayerEvent derive macro

Export the AppLayerEvent derive macro so plugin (or library code) can
use it as expected, for example:

use suricata::applayer::AppLayerEvent;

enum MyEvent {
    EventOne,
    EventTwo,
}

rust/src/applayer.rs

@@ -25,6 +25,10 @@ use std::os::raw::{c_void,c_char,c_int};
 use crate::core::SC;
 use std::ffi::CStr;
 
+// Make the AppLayerEvent derive macro available to users importing
+// AppLayerEvent from this module.
+pub use suricata_derive::AppLayerEvent;
+
 #[repr(C)]
 pub struct StreamSlice {
     input: *const u8,