coresight: core: Make symbol 'csdev_sink' static

The sparse tool complains as follows:

drivers/hwtracing/coresight/coresight-core.c:26:1: warning:
 symbol '__pcpu_scope_csdev_sink' was not declared. Should it be static?

As csdev_sink is not used outside of coresight-core.c after the
introduction of coresight_[set|get]_percpu_sink() helpers, this
change marks it static.
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Link: https://lore.kernel.org/r/20210409094900.1902783-1-weiyongjun1@huawei.comSigned-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>

drivers/hwtracing/coresight/coresight-core.c

@@ -23,7 +23,7 @@
 #include "coresight-priv.h"
 
 static DEFINE_MUTEX(coresight_mutex);
-DEFINE_PER_CPU(struct coresight_device *, csdev_sink);
+static DEFINE_PER_CPU(struct coresight_device *, csdev_sink);
 
 /**
  * struct coresight_node - elements of a path, from source to sink