rust: of: 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>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20260105-define-rust-helper-v2-12-51da5f454a67@google.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: Darksonn

rust/helpers/of.c

@@ -2,7 +2,7 @@
 
 #include <linux/of.h>
 
-bool rust_helper_is_of_node(const struct fwnode_handle *fwnode)
+__rust_helper bool rust_helper_is_of_node(const struct fwnode_handle *fwnode)
 {
 	return is_of_node(fwnode);
 }