nss: fix nssckbi module loading on Windows

- Use .DLL extension instead of .so to load modules on Windows.

Bug: https://curl.haxx.se/mail/lib-2018-09/0077.html
Reported-by: Maxime Legros

Ref: https://github.com/curl/curl/pull/3016/#issuecomment-423069442

Closes https://github.com/curl/curl/pull/3086