Enable precompiled headers for qtwebengine core

Chromium requires us to add a special config to enable precompiled
headers. Note this still depends additionally on that the corresponding
GN flag is also set.

Changes in 3rdparty:
9401dc18ce02 Add precompiled object files to qmake link output
8ba90427ae5e Improve the issue with long file names on windows

Change-Id: I6680232e17a5254d3ca53e5a40e6a15a6b7fc82d
Reviewed-by: Michal Klocek <michal.klocek@qt.io>