fix whitespace errors in acinclude.m4

Use tabs instead of spaces in the sysname lookup case
statement for the macos cases.

Change-Id: Iee03d1b593aee4f6c4bc2488b069b21e116c9f1d
Reviewed-on: http://gerrit.openafs.org/11566
Reviewed-by: D Brashear <shadow@your-file-system.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>