[xml] Make xmlSAXHandler const

["Vegard Nossum" <vegard peltkore net>]

Patch. Added const qualifiers in the parser where functions take an
xmlSAXHandler. This works since the parser doesn't need to change it, and
helps some people by putting their xmlSAXHandler instances in read-only
memory. Does not break existing uses of API. Simple patch follows.

Kind regards,
Vegard Nossum

Attachment: libxml2-2.6.26-const.patch
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