osx: build afscell only for active architecture

osx: build afscell only for active architecture

The InstallerPlugins framework provided by the MacOSX10.12.sdk does not
define symbols for architecture i386. As a result, the OpenAFS code
cannot be built on OS X 10.12.

To fix this problem, build the afscell xcode project only for active
architecture.

Reviewed-on: https://gerrit.openafs.org/12531
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit d39e7c7af77b4e1b043611e1a6e78267f5f956ef)

Change-Id: I34b351b4d383c8dd91f8a7b5b9f35d9aa2854a13
Reviewed-on: https://gerrit.openafs.org/12538
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>