btl/scif: make the exiting member volatile

cmr=v1.8.2:ticket=trac:4626

This commit was SVN r31757.

The following Trac tickets were found above:
  Ticket 4626 --> https://svn.open-mpi.org/trac/ompi/ticket/4626

ompi/mca/btl/scif/btl_scif.h

@@ -91,7 +91,7 @@ typedef struct mca_btl_scif_module_t {
 
     pthread_t listen_thread;
 
-    bool exiting;
+    volatile bool exiting;
 } mca_btl_scif_module_t;
 
 typedef struct mca_btl_scif_component_t {