debian-buster-jenkins: install libelf-dev
Add libelf-dev to be able to build kernel modules with
defconfig_x86_64.
This is an update to commit d6c5c7a0341c966ddef235a5d6e00a536c1a857c,
which should have not just modified the stretch but also the buster
Docker image
Change-Id: I4c442d86688278ff634a286b98637189eef88bfb
Related: OS#5407
