Fix: Register Akismet Anti-Spam as a connector.

Akismet comes with core but the connectors screen was not showing akismet even if akismet was on the file system. This commit fixes the issue.

Props jorgefilipecosta, bluefuton, gziolo.
Fixes #65012.

git-svn-id: https://develop.svn.wordpress.org/trunk@62193 602fd350-edb4-49c9-b593-d223f7449a82