x86_64: Untangle asm/hpet.h from asm/timex.h

When making changes to x86_64 timers, I noticed that touching hpet.h triggered
an unreasonably large rebuild.  Untangling it from timex.h quiets the extra
rebuild quite a bit.

Cc: john stultz <johnstul@us.ibm.com>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>