rust: time: doc: Add missing C header links

The definitions related to jiffies are at linux/jiffies.h, and the
definitions related to ktime_t are at linux/ktime.h, since
`kernel::time` provides the functionality dealing with jiffies and
ktime_t, it makes sense to add links to them from Rust's time module.

Signed-off-by: Boqun Feng <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Reviewed-by: Alice Ryhl <[e-mail]>
Acked-by: Miguel Ojeda <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

rust/kernel/time.rs

···
 //!
 //! This module contains the kernel APIs related to time and timers that
 //! have been ported or wrapped for usage by Rust code in the kernel.
+//!
+//! C header: [`include/linux/jiffies.h`](srctree/include/linux/jiffies.h).
+//! C header: [`include/linux/ktime.h`](srctree/include/linux/ktime.h).
 
 /// The number of nanoseconds per millisecond.
 pub const NSEC_PER_MSEC: i64 = bindings::NSEC_PER_MSEC as i64;