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

'doxygen' package is required to build documentation for SCP-Firmware.
So add it to the list of prerequisite packages to be installed.

Change-Id: I7c152f7399f72023717f550b17bbf7fea5bc2c80
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>