firmware: tegra: include IVC header file only once

Add the necessary definition to prevent compilation
errors from the ivc.h file being included multiple times.
This does not currently cause any compilation issues,
but fix this anyway.

Signed-off-by: Manish Bhardwaj <mbhardwaj@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>