sgi/install_prerequisites.sh: fix add-apt-repository

add-apt-repository command fails in Ubuntu 18.04. To resolve this,
install software-properties-common before invoking the
add-apt-repository command.

In addition to that, add -y to add-apt-repository command to prevent
the script from waiting for user input.

Signed-off-by: Joel Goddard <joel.goddard@arm.com>
Change-Id: I6572cb6ee3954f1d206212640c93002258e09cf4