Add WM2Activities to supported properties

We have the property, so let's announce that we support it.

REVIEW: 120227

netinfo.cpp

@@ -110,6 +110,9 @@ RootInfo *RootInfo::create()
         NET::WM2FullPlacement |
         NET::WM2FullscreenMonitors |
         NET::WM2KDEShadow;
+#if KWIN_BUILD_ACTIVITIES
+        properties2 |= NET::WM2Activities;
+#endif
     const NET::Actions actions = NET::ActionMove |
         NET::ActionResize |
         NET::ActionMinimize |