Move missing piece to required visibility

This commit was SVN r27380.

opal/mca/hwloc/base/base.h

@@ -85,6 +85,7 @@ typedef struct {
     int cntr;
 } opal_hwloc_print_buffers_t;
 opal_hwloc_print_buffers_t *opal_hwloc_get_print_buffer(void);
+extern char* opal_hwloc_print_null;
 OPAL_DECLSPEC char* opal_hwloc_base_print_locality(opal_hwloc_locality_t locality);
 
 #if OPAL_HAVE_HWLOC

opal/mca/hwloc/base/hwloc_base_open.c

@@ -307,7 +307,7 @@ int opal_hwloc_base_open(void)
 
 static bool fns_init=false;
 static opal_tsd_key_t print_tsd_key;
-static char* opal_hwloc_print_null = "NULL";
+char* opal_hwloc_print_null = "NULL";
 
 static void buffer_cleanup(void *value)
 {