x86/hpet: Make boot_hpet_disable extern

HPET on some platform has accuracy problem. Making
"boot_hpet_disable" extern so that we can runtime disable
the HPET timer by using quirk to check the platform.

Signed-off-by: Feng Tang <feng.tang@intel.com>
Cc: Clemens Ladisch <clemens@ladisch.de>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1398327498-13163-1-git-send-email-feng.tang@intel.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>