panel-volume: build fixes   - add missing QMetaType include   - add define for PA_VOLUME_UI_MAX, which is only defined since     pulseaudio 0.9.23
panel-volume: build fixes
  - add missing QMetaType include
  - add define for PA_VOLUME_UI_MAX, which is only defined since
    pulseaudio 0.9.23

DrTobe/lxqt-panel,jubalh/lxqt-panel,rysiowski/lxqt-panel,Ilya87/lxqt-panel,rbazaud/lxqt-panel,jubalh/lxqt-panel,stefonarch/lxqt-panel,pmattern/lxqt-panel,zvacet/lxqt-panel,lxde/lxqt-panel,rysiowski/lxqt-panel,zjes/lxqt-panel,hagabaka/lxqt-panel,jeremywh7/lxqt-panel,cferr/lxqt-panel,ThomasVie/lxqt-panel-de,musarder/lxqt-panel,stefonarch/lxqt-panel,pmattern/lxqt-panel,musarder/lxqt-panel,rbazaud/lxqt-panel,npmiller/lxqt-panel,cferr/lxqt-panel,zvacet/lxqt-panel,zjes/lxqt-panel,npmiller/lxqt-panel,Ilya87/lxqt-panel,DrTobe/lxqt-panel,jeremywh7/lxqt-panel,hagabaka/lxqt-panel,ThomasVie/lxqt-panel-de,lxde/lxqt-panel,stefonarch/lxqt-panel