x86: fix unused variable warning in arch/x86/kernel/hpet.c

Impact: fix build warning

this warning:

  arch/x86/kernel/hpet.c:36: warning: ‘hpet_num_timers’ defined but not used

Triggers because hpet_num_timers is unused in the !CONFIG_PCI_MSI case.

Signed-off-by: Ingo Molnar <mingo@elte.hu>