vendor/expat: import 2.6.3

[ngie]

Summary

The 2.6.1 - 2.6.3 releases address security issues.

The release notes for the 2.6.3 release can be found at
https://github.com/libexpat/libexpat/blob/R_2_6_3/expat/Changes .

[ngie]

I didn't mean to open up this revision.