NetBSD: catch up to <sys/simplelock.h> removal in NetBSD 6.99.x

Change-Id: I2c663fc426914e978e98c6003419503b57a020d3
Reviewed-on: http://gerrit.openafs.org/11087
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>