Do not expose afs_assert.h from other public headers

afs_assert.h redefines the standard assert macro, which is evil and
breaks some applications that might want to include our public headers
(e.g. some versions of Cython).  This was fixed on master by commit
cac74242728ad97e3ce9cef0a949d58c237250f6, which removes afs_assert.h
entirely and adds opr_Assert.  Since that patch may be too invasive
for 1.6.x, here’s a minimal patch that just stops exposing
afs_assert.h from our other public headers.

Change-Id: I39a7b9ae8d43cfe0059e10e47ce4b1c22e01c544
Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Reviewed-on: http://gerrit.openafs.org/10096
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>