afsutil: fix prototype

the prototype for afs_config_get_string was broken. fix it.
1.6 fix only; on master, hcrypto provides this

Change-Id: Id7f877c197e44437fe41cb89630ce6e1bf2d6e3d
Reviewed-on: http://gerrit.openafs.org/9362
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>