I was messing with installing Virtualbox 5.2.18 this morning. Upon attempting to compile the needed kernel modules I was given errors. As it turns out I needed to install the elfutils-libelf-devel.x86_64 package. If you are having trouble installing and running Virtualbox 5.2.18 on Fedora 28 Linux then use the following command to install the elfutils-libelf-devel.x86_64 package.
sudo dnf install elfutils-libelf-devel
After the above package gets installed it should be smooth sailing.