x86/vdso: Remove unused include

Including hrtimer.h is not required and is probably a historical
leftover. Remove it.

Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Link: https://lore.kernel.org/r/20240701-vdso-cleanup-v1-5-36eb64e7ece2@linutronix.de