Fix build error
6547a8c4 introduced dependency on QPointer
but the header was not included, resulting in the following build error:
src/kcommandbar.cpp:494:27: error: implicit instantiation of undefined template 'QPointer<KCommandBar>'
QPointer<KCommandBar> crashGuard = q;
^
/usr/include/qt6/QtGui/qevent.h:36:29: note: template is declared here
template <typename T> class QPointer;
^