This is built in. If you already use Kali's repos, you don't need to get it from an external source:
$ sudo apt install kali-archive-keyring
I'm using a system that adds Kali to a Debian base, so I had to specify the release. You can check that as follows:
$ apt policy kali-archive-keyring
Installed: 2015.2
Candidate: 2015.2
Version table:
2018.1 1
1 http://http.kali.org/kali kali-rolling/main amd64 Packages
1 http://http.kali.org/kali kali-rolling/main i386 Packages
*** 2015.2 100
100 /var/lib/dpkg/status
$ sudo apt install kali-archive-keyring/kali-rolling
This way, you don't have to blindly trust that you're importing the right key in order to prevent a man-in-the-middle attack since the new key is signed by the old one within the repository.
Update: Here's an /etc/apt/sources.list.d/kali.list and /etc/apt/preferences.d/kali-repos.pref, designed to be secondary to another distribution (like Debian), that'll avoid this issue in the future.
If you don't already have this repository and therefore can't get this update, you have two choices:
1: Go to https://http.kali.org/kali/pool/main/k/kali-archive-keyring/, download the .deb file, and install it via dpkg -i kali-archive-keyring*.deb
2: Add it via the repository anyway (it's "insecure" until you add it, though you're at least protected by the HTTPS download in the prior step):
$ sudo apt update -oAcquire::AllowInsecureRepositories=true
$ sudo apt install kali-archive-keyring
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following NEW packages will be installed:
kali-archive-keyring
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 7,008 B of archives.
After this operation, 17.4 kB of additional space will be used.
Do you want to continue? [Y/n]
WARNING: The following packages cannot be authenticated!
kali-archive-keyring
Install these packages without verification? [y/N] y
…
You're now synced with the first part of this answer. Consider using the kali.list and kali-repos.pref I linked there so you don't repeat this problem.