[printer] ensures labels are not put on declarations

Fixes #617

Technically, the C standard distinguishes between declarations and statements,
and only the latter may have labels attached to them. Thus, printing a label
directly over a `Local_init` will result in ill-formed C code. In order to
avoid that, we add a dummy nop (aka `;`) in-between.

A first version was done in !1518, but failed to take into account
`UnspecifiedSequence` whose first element happens to be a `Local_init`