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: NWei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: NStephan Mueller <smueller@chronox.de>
Acked-by: NNeil Horman <nhorman@tuxdriver.com>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>

crypto/algif_rng.c

@@ -182,7 +182,7 @@ static int __init rng_init(void)
 	return af_alg_register_type(&algif_type_rng);
 }
 
-void __exit rng_exit(void)
+static void __exit rng_exit(void)
 {
 	int err = af_alg_unregister_type(&algif_type_rng);
 	BUG_ON(err);