contrib/check-whitespace: Use / for all platform

[inkydragon]

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.

julia/contrib/check-whitespace.jl

Line 33 in 649982a

for path in eachline(`git ls-files -- $patterns`)

"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.

julia/contrib/check-whitespace.jl

Lines 21 to 29 in 649982a

	allow_tabs(path) =
	path == "Make.inc" ||
	endswith(path, "Makefile") ||
	endswith(path, ".make") ||
	endswith(path, ".mk") ||
	startswith(path, joinpath("src", "support")) ||
	startswith(path, joinpath("src", "flisp")) ||
	endswith(path, joinpath("test", "syntax.jl")) ||
	endswith(path, joinpath("test", "triplequote.jl"))

Change: just use / for all platforms.