make "gcc -Werror" happy

Change-Id: I6dd1bf6b4535bb956cb7e85908312d52749a6f2b

forms/source/component/ListBox.cxx

@@ -940,7 +940,7 @@ namespace frm
                         if(*aBoundColumn == -1)
                         {
                             // the type of i matters! It will be the type of the ORowSetValue pushed to aValueList!
-                            for(sal_Int16 i=0; i < aDisplayList.size(); ++i)
+                            for(sal_Int16 i=0; static_cast<ValueList::size_type>(i) < aDisplayList.size(); ++i)
                             {
                                 aValueList.push_back(i);
                             }