hwrng: make symbol 'optee_rng_id_table' static

Fixes the following sparse warning:

drivers/char/hw_random/optee-rng.c:265:35: warning:
 symbol 'optee_rng_id_table' was not declared. Should it be static?

Fixes: 5fe8b1cc ("hwrng: add OP-TEE based rng driver")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>