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/gendsa.c b/apps/gendsa.c
index 2be3b9a..5dacf1b 100644
--- a/apps/gendsa.c
+++ b/apps/gendsa.c
@@ -53,7 +53,7 @@
     char *outfile = NULL, *passoutarg = NULL, *passout = NULL, *prog;
     OPTION_CHOICE o;
     int ret = 1, private = 0;
-    BIGNUM *p = NULL;
+    const BIGNUM *p = NULL;
 
     prog = opt_init(argc, argv, gendsa_options);
     while ((o = opt_next()) != OPT_EOF) {