Bump src/external/tinyxml2 from a0f66fd to 8a519a5

Bumps [src/external/tinyxml2](https://github.com/leethomason/tinyxml2) from `a0f66fd` to `8a519a5`.
- [Release notes](https://github.com/leethomason/tinyxml2/releases)
- [Commits](leethomason/tinyxml2@a0f66fd...8a519a5)

---
updated-dependencies:
- dependency-name: src/external/tinyxml2
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>