Google Maps/Streetview is your friend. The road leading up to the airport is a regional highway (ER) and therefore not prohibited to pedestrians. Taking a cursory look at streetview and satellite views, you can see there is a footpath the whole way, if you approach it from the underpass below ER-101 (right by the memorial of victims from the plane crash of TP425 in 1977 - yikes!) and then take the turnoff to the cargo terminal, turn onto Rua da Manutenção and onto Avenida Marginal which will lead you all the way to the terminal.