XCB: Qt packages are built without session management support

The Library libsm (package libsm-dev) needs to be installed for this to work. Otherwise, notification of session logouts via signal QGuiApplication::commitDataRequest do not work .
An empty QGuiApplication::sessionId() indicates that.

Steps to reproduce

• Run the example
• Expected: A non-empty sessionId is logged and session logoff causes a log entry for commitDataRequest
• Actual

  C++, Qt 5.15.4 xcb fusion () 
   QT_CONFIG[sessionmanager] sessionId="", sessionKey="" 
  Logging to  log.txt