Missing build dependency on libxml2 for mpich package

[johannesring]

@alogg reported today the following error when building mpich:

2015/03/25 07:49:43 - INFO: [package:run_job] topology-xml-libxml.c:16:10: fatal error: 'libxml/parser.h' file not found

The fix was to add libxml2 to the build dependencies for mpich. I will prepare a pull request.

[memmett]

I think the mpich code is just missing some ifdef guards. Perhaps this
would be better addressed upstream instead? @ahmadia and I stumbled upon
this in November.

[johannesring]

Fixed in #702.