Fix typo

The file is KeyFileExt, not KeyFileEx.

Change-Id: Iffd5edc8a60b1a49441822d26dc44fa252bd70e3
Reviewed-on: http://gerrit.openafs.org/10084
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>