x86: HPET: fix sparse warning

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>

arch/x86/kernel/hpet.c

@@ -33,7 +33,7 @@
  * HPET address is set in acpi/boot.c, when an ACPI entry exists
  */
 unsigned long				hpet_address;
-unsigned long				hpet_num_timers;
+static unsigned long			hpet_num_timers;
 static void __iomem			*hpet_virt_address;
 
 struct hpet_dev {