Require NSS for linux builds

OpenSSL certificate validation was dropped in
https://codereview.chromium.org/2862543003

Bundled NSS was deopped in
https://codereview.chromium.org/1882433002



Task-number: QTBUG-52193
Task-number: QTBUG-62891
Change-Id: I300c13c30a2625bbe56beb0659107d2a395b0a4d
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>