[libcalamaresui] Make sure finished() is processed once

- Avoid races which might double-delete the timer, or enter
   the if twice (which would lead to duplicate emissions of
   requirementsComplete and the associated UI glitches).

src/libcalamaresui/modulesystem/RequirementsChecker.cpp

@@ -99,6 +99,9 @@ RequirementsChecker::run()
 void
 RequirementsChecker::finished()
 {
+    static QMutex finishedMutex;
+    QMutexLocker lock( &finishedMutex );
+
     if ( m_progressTimer && std::all_of( m_watchers.cbegin(), m_watchers.cend(), []( const Watcher *w ) { return w && w->isFinished(); } ) )
     {
         cDebug() << "All requirements have been checked.";