botan: Use strongSwan's RNG interface in Botan plugin

This allows using rng_t implementations provided by other plugins to
serve as RNG for Botan.

Closes strongswan/strongswan#192.

src/libstrongswan/plugins/botan/botan_ec_diffie_hellman.c

@@ -205,7 +205,7 @@ botan_ec_diffie_hellman_t *botan_ec_diffie_hellman_create(
 			return NULL;
 	}
 
-	if (botan_rng_init(&rng, "user"))
+	if (!botan_get_rng(&rng, RNG_STRONG))
 	{
 		free(this);
 		return NULL;