autoninja.py: Detect when use_siso=false exists after use_siso=true
I ran into this from having use_siso=true in an include(), and then
trying to turn it off again via an assignment
Change-Id: Iba70ade68c7a3699755d6a0070816283e0538c6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5769918
Commit-Queue: Junji Watanabe <jwata@google.com>
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Junji Watanabe <jwata@google.com>