target: make device_mutex and device_list static

Variables device_mutex and device_list static are local to the source,
so make them static.

Cleans up sparse warnings:
"symbol 'device_list' was not declared. Should it be static?"
"symbol 'device_mutex' was not declared. Should it be static?"
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Signed-off-by: NNicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_device.c

@@ -49,8 +49,8 @@
 #include "target_core_pr.h"
 #include "target_core_ua.h"
 
-DEFINE_MUTEX(device_mutex);
-LIST_HEAD(device_list);
+static DEFINE_MUTEX(device_mutex);
+static LIST_HEAD(device_list);
 static DEFINE_IDR(devices_idr);
 
 static struct se_hba *lun0_hba;