Return the list of all source/header files in `c/` directory.

    The files will have pathnames relative to the current folder, for example
    "c/csv/reader_utils.cc".

def get_files():
    """
    Return the list of all source/header files in `c/` directory.

    The files will have pathnames relative to the current folder, for example
    "c/csv/reader_utils.cc".
    """
    sources = []
    headers = ["datatable/include/datatable.h"]
    assert os.path.isfile(headers[0])
    for dirpath, _, filenames in os.walk("c"):
        for f in filenames:
            fullname = os.path.join(dirpath, f)
            if f.endswith(".h") or f.endswith(".inc"):
                headers.append(fullname)
            elif f.endswith(".c") or f.endswith(".cc"):
                sources.append(fullname)
    return (sources, headers)