[xml] Get rid of a likely typo in xpath.c

[Marius Wachtler <malloc inode at>]

Hello

My compiler is warning me about a likely typo in
xpath.c:xmlXPathCompOpEvalFirst()

#ifdef XP_OPTIMIZED_FILTER_FIRST
       case XPATH_OP_FILTER:
               total =+ xmlXPathCompOpEvalFilterFirst(ctxt, op, first);
           return (total);
#endif

I think the =+ should be +=, however "total" is zero initialized so
this should not make any difference.

-- Marius Wachtler