perf/core: Make local function perf_pmu_snapshot_aux() static

Fixes the following W=1 kernel build warning:

  kernel/events/core.c:6670:6: warning: no previous prototype for 'perf_pmu_snapshot_aux' [-Wmissing-prototypes]
Signed-off-by: Haocheng Xie <xiehaocheng.cn@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20210527031947.1801-2-xiehaocheng.cn@gmail.com

kernel/events/core.c

@@ -6674,10 +6674,10 @@ static unsigned long perf_prepare_sample_aux(struct perf_event *event,
 	return data->aux_size;
 }
 
-long perf_pmu_snapshot_aux(struct perf_buffer *rb,
-			   struct perf_event *event,
-			   struct perf_output_handle *handle,
-			   unsigned long size)
+static long perf_pmu_snapshot_aux(struct perf_buffer *rb,
+                                 struct perf_event *event,
+                                 struct perf_output_handle *handle,
+                                 unsigned long size)
 {
 	unsigned long flags;
 	long ret;