Fix sparse warning:

drivers/scsi/gdth.c:332:5: warning:
 symbol '__gdth_execute' was not declared. Should it be static?

Link: https://lore.kernel.org/r/1586276474-34480-1-git-send-email-wanghai38@huawei.comReported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>