Protect against multiple definitions of typedef pbs_net_t

Current head won't compile on RHEL6.  It complains about multiple definitions of pbs_net_t

In file included from ../../src/include/pbs_job.h:94,
                 from ../../src/include/batch_request.h:105,
                 from node_func.c:32:
../../src/include/server_limits.h:186: error: redefinition of typedef ‘pbs_net_t’
node_func.h:7: note: previous declaration of ‘pbs_net_t’ was here