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-primary.h

@@ -9,4 +9,5 @@
 extern void do_primary_register(void);
 extern void do_primary_unregister(void);
 
+extern void do_primary_help(void);
 #endif