Qt5 dependency with private_headers misses `mkspecs` include path

[David Fort]

Describe the bug
I'm porting a QPA project to meson (qfreerdp) so I need to have access to Qt private headers. I'm detecting all these with :

qt5_deps = dependency('qt5', 
    modules: ['Core', 'Gui', 'Widgets', 'FontDatabaseSupport', 'EventDispatcherSupport', 'ThemeSupport', 'DBus'],
    private_headers: true
)

But when building I have compile errors because of missing qplatformdefs.h file and it seems like it's because no mkspecs directory in the generated include path. If I manually add /usr/lib/x86_64-linux-gnu/qt5/mkspecs/linux-g++-64 in the include_directories the build works.

To Reproduce

Any project that does

#include <QtGui/private/qguiapplication_p.h>

should trigger the bug.

Expected behavior
The include path should have a mkspecs entry.

system parameters
This is a native build on Ubuntu 22.04, meson 1.4.0.