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: YueHaibing <yuehaibing@huawei.com>
Message-Id: <20190320133505.21984-1-yuehaibing@huawei.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>