rust: wait: Add __rust_helper to helpers

This is needed to inline these helpers into Rust code.

Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://patch.msgid.link/20260105-define-rust-helper-v2-25-51da5f454a67@google.com

rust/helpers/wait.c

diff --git a/rust/helpers/wait.c b/rust/helpers/wait.c
index ae48e33..2dde1e4 100644
--- a/rust/helpers/wait.c
+++ b/rust/helpers/wait.c
@@ -2,7 +2,7 @@
 
 #include <linux/wait.h>
 
-void rust_helper_init_wait(struct wait_queue_entry *wq_entry)
+__rust_helper void rust_helper_init_wait(struct wait_queue_entry *wq_entry)
 {
 	init_wait(wq_entry);
 }