scons: Change an = to a += when accumulating sources from filters.

scons: Change an = to a += when accumulating sources from filters.

The loop accidentally used a = when it should have used a +=, meaning
only the sources from the final filter would be used.

Change-Id: Ie066a5f85696f05d9ad3cf61f928b12deb39475b
Reviewed-on: https://gem5-review.googlesource.com/c/16285
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>