[C] Add .gitattributes to normalize line endings. Fixes BUKKIT-3797

According to the coding conventions at https://github.com/Bukkit/CraftBukkit/blob/master/README.md:

* No CRLF line endings, LF only, put your gits 'core.autocrlf' on 'true'.

~~However, there is one file with CRLF line endings. This PR fixes that, and also adds .gitattributes to force LF line endings on all text files.~~ This PR enables autocrlf in .gitattributes, enforcings the README.md recommendation to "put your gits 'core.autocrlf' on 'true'.", without requiring users manually configure their git settings.

Prevents mixed line-endings, originally noticed with n.m.s. BlockJukebox CRLFs (although this specific file was removed in the 1.5 update, this setting will prevent the problem from possibly occurring again in the future.)

Leaky ticket: https://bukkit.atlassian.net/browse/BUKKIT-3797