Adding easyrdf to .gitignore to prevent noise from checkout

Checking out all WMF extensions will put it at extensions/Wikibase/easyrdf
https://gerrit.wikimedia.org/r/p/mediawiki/extensions/Wikibase/easyrdf.git

Change-Id: Id5e21b66bdee37f9b4e25a0e88afafe323f1c9ab