WINGs: Properly mark 'const' more 'char*' in the public API

Because the previous patch brought a (welcome) change in the public API,
seize the opportunity to go further in the improvement.

WINGs/selection.c

@@ -7,7 +7,7 @@
 
 #define MAX_PROPERTY_SIZE 8*1024
 
-char *WMSelectionOwnerDidChangeNotification = "WMSelectionOwnerDidChange";
+const char *WMSelectionOwnerDidChangeNotification = "WMSelectionOwnerDidChange";
 
 typedef struct SelectionHandler {
 	WMView *view;