[xml] some compiler warnings in MSVC

[<oliverst online de>]

When I compile the libxml2-2.6.5 with MSVC6 I get 4 compiler wranings,
that are both caused by the same variable.
It's "xmlParserCtxtPtr->nsTab".

It occurs in parser.c in line 857 in the call of the xmlRealloc
function. And in parserInternals.c in line 1689 at the call of xmlFree.
In both cases it says, that the const identifers differ and that the
pointertype of the first parameter does not match the declaration. I
guess MSVC says that because the given var is a unsigned char** and not
a unsigned char* as expected.

I also get a warning in xpath.c in line 8798, saying, that nothing in
the code refers to "eval_predicates". I checked the code and it's just
called in LIBXML_XPTR_ENABLED sepcific code. maybe the anchor could also
be moved in such a define (such a block is just below the anchor, so you
just have to move it a bit up).

Just wanted to point it out, as those warnings are there since a few
versions and weren't there in older version (2.5.x IIRC).

So long
Oliver