install_prerequisites: include additional packages to install

"libssl-dev" and "libelf-dev" were found to be required to build linux
kernel on AArch64 host machines. So include them in the list of packages
to be installed as prerequisites.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: Id0a1a00968b9180c6f46fa4441fb2dafc8e6a14b