don't know if this helps but vbox and vbox extensions pack need to be same ver I think they are both up to 5.1.10 now
and hopefully your sources.list just has these two lines
deb
http://http.kali.org/kali kali-rolling main contrib non-free
deb-src
http://http.kali.org/kali kali-rolling main contrib non-free
if the above is correct did you do apt-get update && apt-get upgrade -y
and don't do dist-upgrade lately that blows up the desktop but to get to 4.8 kernel this worked for me apt-get install -y 4.8.0-kali1-amd64
then restart kali ... uname -r should be at 4.8 now
then I installed the 4.8 Linux headers you can list them with apt-cache search headers | grep -I Linux
then I installed DKMS with apt-get install -y dkms linux-headers-$(uname -r)
then inserted the guest additions cd copied the VBoxLinuxAdditions.run to my desktop then in terminal went to Desktop and did ./VBoxLinuxAdditions.run
guest additions worked for me then