xwayland: Pass absolute Xwayland executable pass to QProcess

It's a common coding convention in KDE to improve security. Given how
kwin runs, it doesn't matter, but let's do it anyway for the consistency
sake.

src/xwayland/xwaylandlauncher.cpp

@@ -187,7 +187,7 @@ bool XwaylandLauncher::start()
     }
 
     m_xwaylandProcess = new QProcess(this);
-    m_xwaylandProcess->setProgram(QStringLiteral("Xwayland"));
+    m_xwaylandProcess->setProgram(QStandardPaths::findExecutable("Xwayland"));
     m_xwaylandProcess->setArguments(arguments);
     m_xwaylandProcess->setProcessChannelMode(QProcess::ForwardedErrorChannel);
     m_xwaylandProcess->setProcessEnvironment(env);