This way we do not abort a signature verification when we see a

<DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"/>

XML node. Note that this just extends the glue layer, both NSS and
libxmlsec itself already supported SHA-256 already.

Change-Id: I68de99578b839bd7eaa8f21af903aa924c892799