setup.py: add setuptools_git_ls_files to setup_requires
setup.py: add setuptools_git_ls_files to setup_requires

https://github.com/anthrotype/setuptools_git_ls_files

uses git ls-files --recurse-submodule to include all the submodules in the sdist archive.
setuptools_scm has a similar feature, but it currently doesn't support submodules.
We need to include skia submodule in the source distribution to be able to
build from it.

fonttools/skia-pathops