sgi/install_sgi_prerequisites: add 'pkg-config' and 'gdisk' to the list

Add 'pkg-config' and 'gdisk' to the list of list of prerequisite
packages to be installed.

Change-Id: Id11c91b9ddefbae86ee3ac63fe9e27fd49da2717
Reported-by: Toshihisa Oishi <toshihisa.oishi@arm.com>
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>