Deal with the consequences of constifying getters

Reviewed-by: Stephen Henson <steve@openssl.org>
Reviewed-by: Emilia Käsper <emilia@openssl.org>
diff --git a/apps/rsa.c b/apps/rsa.c
index 3a1195f..63e88e6 100644
--- a/apps/rsa.c
+++ b/apps/rsa.c
@@ -214,7 +214,7 @@
     }
 
     if (modulus) {
-        BIGNUM *n;
+        const BIGNUM *n;
         RSA_get0_key(rsa, &n, NULL, NULL);
         BIO_printf(out, "Modulus=");
         BN_print(out, n);