contrib: qt: always add includedir to the .pc file

The generated .pc files do not contain -I${includedir}, only
-I${includedir}/$QTMODULE. This is because QMake finds out that our
standard contrib include path is already in the QMAKE_DEFAULT_INCDIRS
list.

When this includepath is not forced externally through some script and
only the .pc file flags are used for the qt plugin, the build fails:

In file included from /vlc/contrib/x86_64-linux-gnu/include/QtWidgets/QApplication:1,
                 from ../modules/gui/qt/vlc-qt-check.cpp:26:
/vlc/contrib/x86_64-linux-gnu/include/QtWidgets/qapplication.h:43:10: fatal error: QtWidgets/qtwidgetsglobal.h: No such file or directory
   43 | #include <QtWidgets/qtwidgetsglobal.h>
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fixes #27588.