FBSD: initialize 'retval' for afs3_syscall

In the same way as linux_ret.

An ugly hack, but retval is not really used for anything relevant at
the moment, and the compiler will warn about it being used uninitialized
otherwise.

Change-Id: Ia31ea6668ac3bc2edbec143d0b839f3e797ff424
Reviewed-on: http://gerrit.openafs.org/11372
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>