Add cache/ to .gitignore

Add cache/ to .gitignore

On fenari we've always got:

> Untracked files:
>   (use "git add <file>..." to include in what will be committed)
>
>	cache/l10n/
>	cache/message-timestamp

And on my localhost also several files and stuff extensions have
put in there. Might as well ignore cache entirely instead of
just .cdb files.

Change-Id: I3c594a0f746341687c2a35eed5dc2c2b91e68265