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.

Change-Id: I2a2bd5694826b668fceb7402567fba1d0f128479
Reviewed-on: https://gerrit.openafs.org/12531
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>