Build failure: gcc: multiple definition of 'oc_help_info'

Failure occurred under GCC 11:

` /usr/bin/ld: harness.o:/home/virt/projects/occam/kroc/tools/occ21/misc/harness.c:567: multiple definition of 'oc_help_info'; version.o:/home/virt/projects/occam/kroc/tools/occ21/version.c:39: first defined here`

Adding `extern` to `harness.c` fixes the problem.