ARM: perf: avoid exposing internal stop function for v6 PMU

Unlike other pmu functions, armv6pmu_pmu_stop is not declared static.
This patch adds the missing keyword.
Acked-by: NJamie Iles <jamie.iles@jamieiles.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>

arch/arm/kernel/perf_event.c

@@ -1121,7 +1121,7 @@ armv6pmu_start(void)
 	spin_unlock_irqrestore(&pmu_lock, flags);
 }
 
-void
+static void
 armv6pmu_stop(void)
 {
 	unsigned long flags, val;