Add placeholder functions for "repmgr $command --help"

There are now too many options to sensibly fit into general --help
output; we'll add separate output for each repmgr command, e.g.
"repmgr node --help".

repmgr-action-cluster.h

@@ -35,5 +35,6 @@ extern void do_cluster_event(void);
 extern void do_cluster_crosscheck(void);
 extern void do_cluster_matrix(void);
 
+extern void do_cluster_help(void);
 
 #endif