Fix debug-only build on Windows

Do not attempt to build the release version of QtWebEngineProcess if Qt
is not configured for release.

Task-number: QTBUG-53240
Change-Id: Iff52a5049b3eefdd52c405fb80095a4d53c55fba
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>
Reviewed-by: Michal Klocek <michal.klocek@theqtcompany.com>