Add private include paths

[akiselev]

I'm trying to bind QT Advanced Docking System but i'm getting the following error:

Qt-Advanced-Docking-System/src/ads_globals.cpp:45:10: fatal error: qpa/qplatformnativeinterface.h: No such file or directory
    45 | #include <qpa/qplatformnativeinterface.h>
       |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

That header is in QtGui/6.7.0/QtGui/qpa/qplatformnativeinterface.h so I have a very hacky solution below that builds, but it'd be nice to have official support.

let version: qt_build_utils::SemVer = qtbuild.version().clone();
for path in qtbuild.include_paths() {
    let path = path.to_str().unwrap();
    if path.contains("QtGui") {
        cc.include(format!("{}/{}/QtGui", path, version));
    }
}