test: skip buserror test when SIGBUS is not defined in perl POSIX module

test: skip buserror test when SIGBUS is not defined in perl POSIX module

Older versions of the perl POSIX module do not define the SIGBUS symbol, which
causes the opr/softsig-t perl test to fail to compile.  Instead of trying to
defined SIGBUS, which may be platform dependent, skip the buserror unit test on
these older platforms.

Change-Id: Ib8cfd77215ea43566e9d47b501d4989556b83734
Reviewed-on: http://gerrit.openafs.org/12186
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>