fix compilation with clang
not reported to upstream (no bugtracker)
--- a/source/qm_itemlist.h
+++ b/source/qm_itemlist.h
@@ -49,7 +49,7 @@ public:
     QString sorter;
 
     // Override the < operator for alist.sort();
-    bool operator < (const qm_listItemInfo& litem)
+    bool operator < (const qm_listItemInfo& litem) const
     {
         return sorter < litem.sorter;
     }