Restore module loading

The module loading feature got broken a while ago, so restore it, but
have it a bit more explicit this time around.

Reviewed-by: Stephen Henson <steve@openssl.org>

apps/prime.c

@@ -109,6 +109,9 @@ int prime_main(int argc, char **argv)
     argc = opt_num_rest();
     argv = opt_rest();
 
+    if (!app_load_modules(NULL))
+        goto end;
+
     if (argc == 0 && !generate) {
         BIO_printf(bio_err, "%s: No prime specified\n", prog);
         goto end;