curl: Enable HTTP Strict-Transport-Security (HSTS) for build in CMake

Upstream curl enabled this by default starting in curl 7.77.  We merged
that version of curl in commit cd40922edb (Merge branch 'upstream-curl'
into update-curl, 2021-05-27, v3.21.0-rc1~120^2~2) but accidentally
switched HSTS off in the build system.  Enable it now.