qt4: singleton: Use vlc_mutex_locker instead of manual lock/unlock
qt4: singleton: Use vlc_mutex_locker instead of manual lock/unlock

xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc