ipmi: Make ipmi_interfaces_srcu variable static

Fix sparse warning:

drivers/char/ipmi/ipmi_msghandler.c:635:20: warning:
 symbol 'ipmi_interfaces_srcu' was not declared. Should it be static?
Signed-off-by: NYueHaibing <yuehaibing@huawei.com>
Message-Id: <20190320133505.21984-1-yuehaibing@huawei.com>
Signed-off-by: NCorey Minyard <cminyard@mvista.com>

drivers/char/ipmi/ipmi_msghandler.c

@@ -635,7 +635,7 @@ static DEFINE_MUTEX(ipmidriver_mutex);
 
 static LIST_HEAD(ipmi_interfaces);
 static DEFINE_MUTEX(ipmi_interfaces_mutex);
-struct srcu_struct ipmi_interfaces_srcu;
+static struct srcu_struct ipmi_interfaces_srcu;
 
 /*
  * List of watchers that want to know when smi's are added and deleted.