Network: Fix gcc warning about conversion from string to wchar *

Fix gcc 4.7 warning:
'deprecated conversion from string constant to 'SEC_WCHAR*'

Change-Id: I5df44ffdc0d505789f99b58d68e02b7535ce761b
Reviewed-by: Qt Doc Bot <qt_docbot@qt-project.org>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>