Has Apple released updates for its Airport base stations to protect against the KRACK vulnerability?
At present there are no updates for the AirPort product line.  Apple is notoriously tight lipped about security issues.


  For our customers' protection, Apple doesn't disclose, discuss, or
  confirm security issues until an investigation has occurred and
  patches or releases are available. Recent releases are listed on the
  Apple security updates page.


If/when Apple releases an update that's security related, it will be listed on the Apple Security Updates page.  You can get more info about this from the question/answer "Has the WiFi KRACK vulnerability been patched for iOS?"

Additionally, you can check the AirPort Firmware Update page which will provide directions, links, and details on the updates and how to apply them.

At the time of this post, the most recent update were (dated Dec. 20, 2016)


7.7.8 for the AirPort Extreme and Time Capsule 802.11ac products
7.6.8 for the Airport Extreme, Express and Time Capsule 802.11n products


Therefore, at the time of this answer/post there have been no updates to address the KRACK vulnerability.