rust: module_param: add missing newline to pr_warn_once

rust: module_param: add missing newline to pr_warn_once

Add a trailing newline ('\n') to the pr_warn_once! call in set_param to
ensure the kernel ring buffer flushes the message correctly and
prevents log line smearing.

Signed-off-by: Kenny Glowner <SisyphusCode0311@gmail.com>
Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://github.com/Rust-for-Linux/linux/issues/1139
[Sami: Updated the commit message as we use pr_warn_once now.]
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>