scons: Remove the extra_deps option from the helper function make_obj.

scons: Remove the extra_deps option from the helper function make_obj.

This had only one use, and made the function more complicated than it
needed to be.

Change-Id: I22147a0c1239b457c577a6a24b423065e539833b
Reviewed-on: https://gem5-review.googlesource.com/5983
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>