gclient: Add aix to host_os list
gclient: Add aix to host_os list

R=mmoss@chromium.org, 6693ee08b084bb2ffa767f664c99b689395e0e4b@chromium.org, phajdan.jr@chromium.org

Bug:
Change-Id: I0bfe0c38f3b1bfa5c8bd40747a0c3483d823cbdc
Reviewed-on: https://chromium-review.googlesource.com/660722
Reviewed-by: Dirk Pranke <6693ee08b084bb2ffa767f664c99b689395e0e4b@chromium.org>
Commit-Queue: Dirk Pranke <6693ee08b084bb2ffa767f664c99b689395e0e4b@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools