[SPIIP-79] Fix 'xmlTextWriterPtr' build issue
The build has been failing in cohfar_accumbackground because 'xmlTextWriterPtr' is undefined. I've added #include <libxml/writer.h> and that seems to fix it.
(Why does this happen when I build and not other people? Open question, but I imagine that there must be some subtle change to the dependencies that controls whether libxml/writer.h was included through one of the other header files.)