.gitignore: Replace all SDL2 with SDL3
Some of these are probably unnecessary now that Autotools (with its
optional in-tree builds) has been deleted in favour of CMake (with
mandatory out-of-tree builds), but let's be consistent.
Spotted while doing a `git grep` for other bugs of the same form as #6801.
Signed-off-by: Simon McVittie <[email protected]>
Simon McVittie