backend: ensure that server side releases/frees shm segment on exit

We were leaking one for every job run.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/fio.h b/fio.h
index 393f381..6a7bd03 100644
--- a/fio.h
+++ b/fio.h
@@ -445,6 +445,7 @@
 extern void fio_unpin_memory(void);
 extern int __must_check allocate_io_mem(struct thread_data *);
 extern void free_io_mem(struct thread_data *);
+extern void free_threads_shm(void);
 
 /*
  * Reset stats after ramp time completes