x86/hpet: make variable 'per_cpu__cpu_bc_channel' static

The variable is only ever used inside the file where it's
defined, therefore it can have static storage. This also
resolves a violation of MISRA C:2012 Rule 8.4 due to the absence
of a declaration prior to the definition.

Fixes: 3ba523ff95 ("CPUIDLE: enable MSI capable HPET for timer broadcast")
Signed-off-by: Nicola Vetrini <nicola.vetrini@bugseng.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/hpet.c

@@ -49,7 +49,7 @@ static struct hpet_event_channel *__read_mostly hpet_events;
 /* msi hpet channels used for broadcast */
 static unsigned int __read_mostly num_hpets_used;
 
-DEFINE_PER_CPU(struct hpet_event_channel *, cpu_bc_channel);
+static DEFINE_PER_CPU(struct hpet_event_channel *, cpu_bc_channel);
 
 unsigned long __initdata hpet_address;
 int8_t __initdata opt_hpet_legacy_replacement = -1;