Move the libxml headers out of the header and into the

Move the libxml headers out of the header and into the
implementation file.

The files that #include osrf_message.h compile just fine
without the nested libxml headers, and there's no reason
to make the compiler paw through them.

M    include/opensrf/osrf_message.h
M    src/libopensrf/osrf_message.c

git-svn-id: svn://svn.open-ils.org/OpenSRF/trunk@1854 9efc2488-bf62-4759-914b-345cdb29e865