Fix build error when VPX support is enabled (closes #71)
Fix build error when VPX support is enabled (closes #71)

Kaffeine/qxmpp,Ilyatk/qxmpp,pol51/QXMPP-CMake,ycsoft/qxmpp,ycsoft/qxmpp,kollix/qxmpp,Kaffeine/qxmpp,gamenet/qxmpp,cccco/qxmpp,cccco/qxmpp,pol51/QXMPP-CMake,pol51/QXMPP-CMake,KangLin/qxmpp,gamenet/qxmpp,gamenet/qxmpp,ycsoft/qxmpp,Ilyatk/qxmpp,Kaffeine/qxmpp,KangLin/qxmpp,LightZam/qxmpp,Ilyatk/qxmpp,olesalscheider/qxmpp,LightZam/qxmpp,olesalscheider/qxmpp,olesalscheider/qxmpp,kollix/qxmpp,cccco/qxmpp,KangLin/qxmpp,kollix/qxmpp,LightZam/qxmpp