Bump ext/SDL from e761770 to a71f99c

Bumps [ext/SDL](https://github.com/libsdl-org/SDL) from `e761770` to `a71f99c`.
- [Release notes](https://github.com/libsdl-org/SDL/releases)
- [Commits](libsdl-org/SDL@e761770...a71f99c)

---
updated-dependencies:
- dependency-name: ext/SDL
  dependency-type: direct:production
...

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