priv: correctly declare `priv_cmd` type

`enum {} priv_cmd` was the declaration of a variable `priv_cmd` as an
anonymous enum. This is not what was expected. Instead, we want `enum
priv_cmd` to be a declaration of a named enum: `enum priv_cmd {}` (we
could also use `typedef enum {} priv_cmd`, but in lldpd, `typedef` is
seldomly used).

When `priv_cmd` was moved out of `priv.c`, its symbol was defined in
several objects. It seems that recent versions of gcc are able to cope
with that (because `priv_cmd` variable was not used, so it was
optimized out I think, not sure). However, some older versions like
4.2.1 complained about duplicate objects during the link phase.