x86_64: fix section mismatch warning in hpet.c

x86_64: fix section mismatch warning in hpet.c

Fix following warnings:
WARNING: vmlinux.o(.text+0x945e): Section mismatch: reference to .init.text:__set_fixmap (between 'hpet_arch_init' and 'hpet_mask_rtc_irq_bit')
WARNING: vmlinux.o(.text+0x9474): Section mismatch: reference to .init.text:__set_fixmap (between 'hpet_arch_init' and 'hpet_mask_rtc_irq_bit')

hpet_arch_init is only used from __init context so mark it __init.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>