Fix system-zlib configure name collision with core

When -zlib is provided on windows for main configure line
the system-zlib feature is forced on webengine. This
is not expected since we assume zlib only for unix builds.
Prepend 'webengine' for webengine system-zlib feature.

Task-number: QTBUG-63367
Change-Id: I983d2e0df0badbaf7a076662ce178009be151406
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>