libxml: Silence several gcc warnings.

libxml: Silence several gcc warnings.

* gnulib-local/lib/libxml/SAX2.c (xmlNsErrMsg): Mark as possibly unused.
(xmlSAX2AttributeDecl, xmlSAX2ElementDecl, xmlSAX2NotationDecl): Mark a local
variable as possibly unused.
* gnulib-local/lib/libxml/encoding.c (xmlCharEncCloseFunc): Likewise.
* gnulib-local/lib/libxml/trionan.c (compute_minus_zerod, minus_zerod): Mark as
possibly unused.
* gnulib-local/lib/libxml/tree.c (xmlDOMWrapCloneNode): Use a differently typed
variable cloneAttr.