fuzz: Correctly sort members initialization to prevent GCC warnings

tests/fuzz/ssh_client_fuzzer.cpp:45:1: error: designator order for field ‘ssh_callbacks_struct::userdata’ does not match declaration order in ‘ssh_callbacks_struct’

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

tests/fuzz/ssh_client_fuzzer.cpp

@@ -40,8 +40,8 @@ static int auth_callback(const char *prompt,
 }
 
 struct ssh_callbacks_struct cb = {
-    .auth_function = auth_callback,
     .userdata = NULL,
+    .auth_function = auth_callback,
 };
 
 static void select_loop(ssh_session session, ssh_channel channel)