Adapt to g_string_free glib 2.75.3 change

glib 2.75.3 changes warning behaviour of `g_string_free()`.
See:
* https://gitlab.gnome.org/GNOME/glib/-/merge_requests/3219
* https://gitlab.gnome.org/GNOME/glib/-/merge_requests/3226

Use this opportunity to replace the use of GString with
`g_strdup_printf()` where possible.
Otherwise correctly take the return value of `g_string_free()`
which is nicer anyways.

src/ui/privwin.c

@@ -286,8 +286,5 @@ privwin_get_string(ProfPrivateWin* privwin)
         g_string_append_printf(res, ", %d unread", privwin->unread);
     }
 
-    char* resstr = res->str;
-    g_string_free(res, FALSE);
-
-    return resstr;
+    return g_string_free(res, FALSE);
 }