Build fix for recent FreeBSD -current

r273707 added a flags argument to syscall_register(), so
add the appropriate version check in param.generic_fbsd.h
and ues that in the main code.

Change-Id: I7ddf6e1295d7ed94625f19fdeee4e38ef5fd511e
Reviewed-on: http://gerrit.openafs.org/11565
Tested-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: D Brashear <shadow@your-file-system.com>