Add nonce for requests verification

[Oren-Hahiashvili]

oceanwp-sticky-header/sticky-header-oceanwp.php

Line 80 in b4f8bec

if (!empty($_POST['oceanwp_header_style']) && in_array($_POST['oceanwp_header_style'], $sticky_header_styles)){

There is a CVE exists without much of details, it would be helpful to use a nonce to verify the incoming POST requests