FBSD: cast lkmnosys appropriately

FBSD: cast lkmnosys appropriately

lkmnosys is a function, and as such has its own prototype which
includes a named struct specific to it (struct nosys_args).  When
comparing its address to an entry in the syscall table, we must
cast it to a sy_call_t to correctly do the comparison, lest gcc
warn us about comparing distinct pointer types without a cast.
This warning recently became an error due to bsd.kmod.mk, so it
causes the build to fail on 8.1 and earlier, which do not use
syscall_register() due to a conflicting entry for our syscall
in syscalls.master.

Change-Id: I606aaf73e433a50ea41adaab842d61ee69653bd5
Reported-by: Garrett Wollman
Reviewed-on: http://gerrit.openafs.org/5087
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Garrett Wollman <wollman@csail.mit.edu>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>