x86/vdso: Move the rng offset to vsyscall.h

vvar.h will go away, so move the last useful bit into vsyscall.h.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20241010-vdso-generic-base-v1-17-b64f0842d512@linutronix.de