Fix regression introduced with If2dc18cabf (SMW_Exp_Element.php)

## Error
Unexpected non-MediaWiki exception encountered, of
type "InvalidArgumentException" exception 'InvalidArgumentException'
with message '$lexicalForm needs to be a string' in
...\export\SMW_Exp_Element.php:249

## Analysis
SMWExpLiteral constructor forces the first parameter to be of type
string where $diWikiPage->getNamespace() is an integer

Change-Id: I21dd6e2c261b5ae7141af30d189320d4f17e095b