aflplusplus: add python3 to makedepends
    
    Without that fix, we have:
        |  [-] You seem to need to install the package python3-dev,
               python2-dev or python-dev (and perhaps python[23]-apt),
               but it is optional so we continue
    
    Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>