qdoc: Remove -fPIC from default options when Q_OS_WIN

This update only adds -fPIC to the options passed to clang
if Q_OS_WIN is not defined.

Change-Id: I473ac14a026aecff141ab6d16e6283efe7bc5bd5
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>