qt5.qtwebkit: Fix paths in pc files

In https://github.com/NixOS/nixpkgs/commit/be1b22538a60e52987d238bb11f93043682b6e9a,
CMake setup-hook started to respect setOutputFlags. Since qtModule disables it by default,
cmake no longer received the multiple-output adjusted installation directory flags when
building the qtwebkit package. This resulted in incorrect paths in generated pkg-config files
and CMake config files.

Let’s temporarily enable setOutputFlags in the qtwebkit expression, until the commit is reverted.