libafs: fix Afs_syscall definition for UKERNEL

For the UKERNEL case, Afs_syscall is prototyped and used as (void).
Make the function definition use the void keyword to avoid a warning
about an old style definition.

Change-Id: Ic22f58a6c646d6f557e5891ce2ed51fba50fba35
Reviewed-on: http://gerrit.openafs.org/9402
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>