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/dsa.c b/apps/dsa.c
index ef50fed..6ac9a40 100644
--- a/apps/dsa.c
+++ b/apps/dsa.c
@@ -195,7 +195,7 @@
     }
 
     if (modulus) {
-        BIGNUM *pub_key = NULL;
+        const BIGNUM *pub_key = NULL;
         DSA_get0_key(dsa, &pub_key, NULL);
         BIO_printf(out, "Public Key=");
         BN_print(out, pub_key);