crypto: algif_rng - fix sparse non static symbol warning

Fixes the following sparse warnings:

crypto/algif_rng.c:185:13: warning:
 symbol 'rng_exit' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Stephan Mueller <smueller@chronox.de>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>