Impact: make global variable static

Fix this sparse warning:

 arch/x86/kernel/hpet.c:36:18: warning: symbol 'hpet_num_timers' was
 not declared. Should it be static?
Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: Ingo Molnar <mingo@elte.hu>