Use << instead of + in SAL_WARN_IF

Change-Id: Iedf02c89954458e762a1997ed1676a278570a00c
Reviewed-on: https://gerrit.libreoffice.org/67251
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

xmloff/source/forms/elementimport.cxx

@@ -889,7 +889,7 @@ namespace xmloff
             SAL_WARN_IF(
                     _rPropValue.Name == PROPERTY_EFFECTIVE_VALUE
                 ||  _rPropValue.Name == PROPERTY_EFFECTIVE_DEFAULT, "xmloff",
-                "OControlImport::implTranslateValueProperty: invalid property type/name combination, Any and " + _rPropValue.Name);
+                "OControlImport::implTranslateValueProperty: invalid property type/name combination, Any and " << _rPropValue.Name);
 
             // Both properties are allowed to have a double or a string value,
             // so first try to convert the string into a number