libspl_assert: always link -lpthread on FreeBSD

The pthread_* functions are in -lpthread on FreeBSD. Some of them are
implicitly linked through libc, but on FreeBSD 13 at least
pthread_getname_np() is not. Just be explicit, since -lpthread is the
documented interface anyway.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #16168