suppress warnings from libxml2

It appears libxml2 uses gtk-doc instead of doxygen and that
clang -Wdocumentation follows include files across libraries.
So our normal third party module warning suppression won't work.

Change-Id: Ic53045a570aa97c77352b5585dacd8cc47342895
Merge topic 'suppress_libxml2_doc_warnings' into master

f62f9fd0 suppress warnings from libxml2