build: update curl submodule to 7.66.0

The submodule was on 7.65.3 version. This update closes two security
problems:

https://curl.haxx.se/docs/CVE-2019-5482.html
https://curl.haxx.se/docs/CVE-2019-5481.html

See also curl-7.66.0 release notes:

https://curl.haxx.se/changes.html#7_66_0

Fixes #4502.