rust: init: use explicit ABI to clean warning in future compilers

Starting with Rust 1.86.0 (currently in nightly, to be released on
2025-04-03), the `missing_abi` lint is warn-by-default [1]:

    error: extern declarations without an explicit ABI are deprecated
        --> rust/doctests_kernel_generated.rs:3158:1
         |
    3158 | extern {
         | ^^^^^^ help: explicitly specify the C ABI: `extern "C"`
         |
         = note: `-D missing-abi` implied by `-D warnings`
         = help: to override `-D warnings` add `#[allow(missing_abi)]`

Thus clean it up.

Link: rust-lang/rust#132397 [1]
Signed-off-by: Miguel Ojeda <[email protected]>

Author: ojeda

Diff for: rust/kernel/init.rs

@@ -854,7 +854,7 @@ pub unsafe trait PinInit<T: ?Sized, E = Infallible>: Sized {
     /// use kernel::{types::Opaque, init::pin_init_from_closure};
     /// #[repr(C)]
     /// struct RawFoo([u8; 16]);
-    /// extern {
+    /// extern "C" {
     ///     fn init_foo(_: *mut RawFoo);
     /// }
     ///