Build of qwaylandinputocntext fails in environments where xkbcommon is not installed

If xkbcommon is not installed, the following files in qt-everywhere-src-5.15.4 fail to build.　

　qtwayland/src/client/qwaylandinputcontext.cpp

The error occurs in the following two lines of QWaylandInputContext::reset().

    if (m_composeState)
        xkb_compose_state_reset(m_composeState);

I think that we probably need to add the following "#if QT_CONFIG(xkbcommon)".

#if QT_CONFIG(xkbcommon)
    if (m_composeState)
        xkb_compose_state_reset(m_composeState);
#endif