Bump externals/rmlui from c6a1b62 to 55d18ed

Bumps [externals/rmlui](https://github.com/mikke89/RmlUI) from `c6a1b62` to `55d18ed`.
- [Release notes](https://github.com/mikke89/RmlUI/releases)
- [Commits](mikke89/RmlUi@c6a1b62...55d18ed)

---
updated-dependencies:
- dependency-name: externals/rmlui
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>