Fix multiple memory leaks related to rosterwin_roster()

There are multiple paths which lead to rosterwin_roster(). The function
doesn't free list returned by wins_get_private_chats().

src/ui/rosterwin.c

@@ -172,6 +172,7 @@ rosterwin_roster(void)
             _rosterwin_private_chats(layout, orphaned_privchats);
         }
         prefs_free_string(privpref);
+        g_list_free(privchats);
         g_list_free(orphaned_privchats);
     }