import-checker: ensure cffi is always a system module
I've had reports that this is not always happening, so whitelist it
the way we whitelist other problem cases.
# See http://EditorConfig.org for the specification
root = true
[*.py]
indent_size = 4
indent_style = space
trim_trailing_whitespace = true
[*.{c,h}]
indent_size = 8
indent_style = tab
trim_trailing_whitespace = true