Build: Add dependency check for SST on ITS

SST calls ITS APIs as its backend filesystem.
So it's impossible to build SST without ITS.
This patch forces to enable ITS when SST is enabled.

Change-Id: I09866c2d96c2b3b64e7c32b556d32ed078098636
Signed-off-by: Kevin Peng <kevin.peng@arm.com>