optee/unit_test: Include optee modules in product unit test

Update optee mbx unit test make file to reflect the new path due to the
recent move of optee files to product/.

Signed-off-by: Wen Ping Teh <wenping.teh@arm.com>
Change-Id: I5569e590eeecc2107c5bdafd50f288e676f494c2