Update libxml gyp and BUILD.gn, was missing some .h files

This caused the analyze step to be too aggressive in skipping things
which caused https://codereview.chromium.org/1204813002 to land and
break many compiles.

(Case-insensitive sort order preserved to make it clear what changes
in this CL, I'll do a separate no-op that sorts the list properly.)

BUG=463958, 502468, 503747

Review URL: https://codereview.chromium.org/1207543002

Cr-Commit-Position: refs/heads/master@{#335822}
2 files changed