gitattributes: force LF line endings for all files

This avoids default CRLF on msysgit for Windows which can cause trouble.
Cf. https://help.github.com/articles/dealing-with-line-endings#text-eollf

Commit dad2ccc9b3b27d3f438eeea53b771cc85f6dfa32 followup.
Closes #7723.