Mention libkrb5-dev dependency

doc/update/6.x-or-7.x-to-7.7.md

@@ -89,6 +89,9 @@ sudo apt-get install logrotate
 
 # Install pkg-config and cmake, which is needed for the latest versions of rugged
 sudo apt-get install pkg-config cmake
+
+# Install Kerberos header files, which are needed for GitLab EE Kerberos support
+sudo apt-get install libkrb5-dev
 ```
 
 ## 5. Configure Redis to use sockets