outbound: fix build on xenial

> outbound_mod.c: In function 'mod_init':
> outbound_mod.c:122:2: error: too many arguments to function 'mod_init_openssl'
>   mod_init_openssl(NULL);
>   ^
> outbound_mod.c:82:13: note: declared here
>  static void mod_init_openssl(void)
             ^

src/modules/outbound/outbound_mod.c

@@ -119,7 +119,7 @@ static int mod_init(void)
 	ob_key.len = OB_KEY_LEN;
 
 #if OPENSSL_VERSION_NUMBER < 0x010101000L
-	mod_init_openssl(NULL);
+	mod_init_openssl();
 #else
 	run_threadV(mod_init_openssl);
 #endif