hydra: slurm hostlist_t should be used as pointer

Appears Slurm changed the signature and usage of its hostlist function
and hostlist_t type. `hostlist_t` is an opaque struct, and should be
used in its pointer form.

src/pm/hydra/lib/tools/bootstrap/external/slurm_query_node_list.c

@@ -26,7 +26,7 @@ static struct HYD_node *global_node_list = NULL;
 #if defined(HAVE_SLURM)
 static HYD_status list_to_nodes(char *str)
 {
-    hostlist_t hostlist;
+    hostlist_t *hostlist;
     char *host;
     int k = 0;
     HYD_status status = HYD_SUCCESS;