zfs_main.c: fix unused variable error with GCC

zfs_setproctitle_init() is stubbed out on FreeBSD.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Ameer Hamza <ahamza@ixsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: Rob Wing <rob.fx907@gmail.com>
Closes #14441