Increase debug symbol level on Windows to generate all Chromium symbols

Debug symbol level 1 is not enough to generate all debug symbols on
Windows. The symbols for the files found in src/core are generated with
symbol_level=1, but no symbols are generated for the files in
src/3rdparty.

Change-Id: I30729981d5956c1c6a8040008bc00af1f19c15d2
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>