contrib/check-whitespace: Use / for all platform (#52468)
When built on Windows, the source code build will not pass
`check-whitespace`.
_check-whitespace issues (truncated):_
```
$ make check-whitespace
Whitespace check found 1658 issues:
src/flisp/aliases.scm:25 -- tab
src/flisp/aliases.scm:27 -- tab
src/flisp/aliases.scm:28 -- tab
src/flisp/aliases.scm:29 -- tab
src/flisp/aliases.scm:73 -- tab
src/flisp/color.lsp:55 -- tab
....
```
Because `git ls-files` use `/` as a path separator on Windows.
https://github.com/JuliaLang/julia/blob/649982aa0995c45165084baa643ce5c7c5b489cb/contrib/check-whitespace.jl#L33
_"git ls-files" output on win (truncated):_
```
$ git ls-files -- *.jl
base/Base.jl
base/Enums.jl
base/abstractarray.jl
base/abstractarraymath.jl
base/abstractdict.jl
base/abstractset.jl
base/accumulate.jl
...
```
But `joinpath` gives `\\` on win, so it won't match.
https://github.com/JuliaLang/julia/blob/649982aa0995c45165084baa643ce5c7c5b489cb/contrib/check-whitespace.jl#L21-L29
Change: just use `/` for all platforms.
