man-pages: create the man3 subdirectory in prep-noistall

man-pages: create the man3 subdirectory in prep-noistall

This should fix a build failure reported on the openafs-devel list
today.

Reviewed-on: https://gerrit.openafs.org/13533
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit e7ea4781f07b29f7f0fc0b5ba17303bd68022e54)

Change-Id: I996aec343f80031c7118e7b6b0d829751a85e228
Reviewed-on: https://gerrit.openafs.org/13535
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>