Tell her in a dramatic and husky voice, like Han Solo with Princess Leia, "I'll wait for you".
Or not.. Perhaps she's opening up now about her feelings because of Africa, and wants to know there's something to come back to? It'll give you time apart to think about each other which will hopefully make the heart fonder for both of you. I would see her up until she goes, and do what comes naturally. If you're feeling the feeling, it's right.