Make EventNotifyFn public

This type alias is used in a public interface, so it should be public. The main
effect of this is that the documentation is prettier. For example,
BootServices::create_event will show a `Option<EventNotifyFn>` argument, rather
than `Option<very-long-function-signature>`.

uefi/src/table/boot.rs

@@ -1616,8 +1616,8 @@ impl<'guid> SearchType<'guid> {
     }
 }
 
-/// Raw event notification function
-type EventNotifyFn = unsafe extern "efiapi" fn(event: Event, context: Option<NonNull<c_void>>);
+/// Event notification callback type.
+pub type EventNotifyFn = unsafe extern "efiapi" fn(event: Event, context: Option<NonNull<c_void>>);
 
 /// Timer events manipulation.
 #[derive(Debug)]