posix-timers: Rename k_itimer:: It_requeue_pending

Prepare for using this struct member to do a proper reprogramming and
deletion accounting so that stale signals can be dropped.

No functional change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/all/20241001083835.611997737@linutronix.de