sgi/distro.sh: source run_model.sh

The run_model.sh script of a platform is not expected to be have execute
permission. So source the run_model.sh file as required.

Signed--off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: I3b625cf3ade2f9e30a74e93f2c8b5d427c11ddd0