drivers/macintosh/smu.c: Fix undeclared symbol warning

Make kernel with `C=2`:
drivers/macintosh/smu.c:1018:30: warning: symbol
'__smu_get_sdb_partition' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang Wensheng <wangwensheng4@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200914122615.65669-1-wangwensheng4@huawei.com