There's however an existing example code on the libcurl site showing how you do FTP-SSL to download a file: ftpsget.c - it shows the very little SSL magic you need to add. That magic is the same for uploads:
/* We activate SSL and we require it for both control and data */ curl_easy_setopt(curl, CURLOPT_USE_SSL, CURLUSESSL_ALL);
In fact, you can add this single line into the FTP upload example. and it'll do FTPS uploads for you.