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

drivers/macintosh/smu.c

@@ -1015,7 +1015,7 @@ static struct smu_sdbp_header *smu_create_sdb_partition(int id)
 /* Note: Only allowed to return error code in pointers (using ERR_PTR)
  * when interruptible is 1
  */
-const struct smu_sdbp_header *__smu_get_sdb_partition(int id,
+static const struct smu_sdbp_header *__smu_get_sdb_partition(int id,
 		unsigned int *size, int interruptible)
 {
 	char pname[32];