sgi/install_prerequisites: add 'xterm' to prerequisite package list

'xterm' package is required by the FVP to display the UART console
output. So add it to the list of prerequisite packages to be installed.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: I7cd246cd9b1058ace58b176f8cf019b1c909dd23