tests: posix signal constants in rx/perf test

Export the posix signal constants in the rx/perf perl test. Fixes a
perl syntax error on solaris.

Change-Id: Iaad361b8533787f9ad97fa00221e01e687f50723
Reviewed-on: http://gerrit.openafs.org/9836
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>