Add missing keysym in qxcbkeyboard.cpp

At both QWindowSystemInterface::handleExtendedKeyEvent calls (in qxcbkeyboard.cpp) the transmission of the native keysym is missing.

Change:

QWindowSystemInterface::handleExtendedKeyEvent(window, time, type, qtcode, modifiers,
code, 0, state, string.left(count), isAutoRepeat);

to:

QWindowSystemInterface::handleExtendedKeyEvent(window, time, type, qtcode, modifiers,
code, sym, state, string.left(count), isAutoRepeat);

and:

QWindowSystemInterface::handleExtendedKeyEvent(window, time, QEvent::KeyPress, qtcode, modifiers,
code, 0, state, string.left(count), isAutoRepeat);

to:

QWindowSystemInterface::handleExtendedKeyEvent(window, time, QEvent::KeyPress, qtcode, modifiers,
code, sym, state, string.left(count), isAutoRepeat);

should fix this. Now QKeyEvent::nativeVirtualKey() should no longer return 0 if any key is pressed.