include/linux/hrtimer.h: Pad hrtimer struct for KABI

Bugzilla: https://bugzilla.redhat.com/2033473
Depends: https://bugzilla.redhat.com/2024595

Upstream Status: RHEL9 only

Pad hrtimer struct.

To make this commit easier to find in future string searches:
RH_KABI kabi rh_kabi.

Signed-off-by: Prarit Bhargava prarit@redhat.com