Fix XMLTABLE on older libxml2

libxml2 older than 2.9.1 does not have xmlXPathSetContextNode (released
in 2013, so reasonable platforms have trouble).  That function is fairly
trivial, so I have inlined it in the one added caller.  This passes
tests on my machine; let's see what the buildfarm thinks about it.

Per joint complaint from Tom Lane and buildfarm.

src/backend/utils/adt/xml.c

@@ -4431,7 +4431,7 @@ XmlTableGetValue(TableFuncScanState *state, int colnum,
 	PG_TRY();
 	{
 		/* Set current node as entry point for XPath evaluation */
-		xmlXPathSetContextNode(cur, xtCxt->xpathcxt);
+		xtCxt->xpathcxt->node = cur;
 
 		/* Evaluate column path */
 		xpathobj = xmlXPathCompiledEval(xtCxt->xpathscomp[colnum], xtCxt->xpathcxt);