Fix ifdefs resulting in dead code

Summary:
Use #else, otherwise there are two return statements after each other
which doesn't make sense.

Reviewers: #kwin, romangg

Reviewed By: #kwin, romangg

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D23068

effects.cpp

@@ -1190,16 +1190,18 @@ QList< int > EffectsHandlerImpl::currentTabBoxDesktopList() const
 {
 #ifdef KWIN_BUILD_TABBOX
     return TabBox::TabBox::self()->currentDesktopList();
-#endif
+#else
     return QList< int >();
+#endif
 }
 
 int EffectsHandlerImpl::currentTabBoxDesktop() const
 {
 #ifdef KWIN_BUILD_TABBOX
     return TabBox::TabBox::self()->currentDesktop();
-#endif
+#else
     return -1;
+#endif
 }
 
 EffectWindow* EffectsHandlerImpl::currentTabBoxWindow() const