Remove '../platform2/' from platform2 relative paths in gyp files.

The paths in platform2 gyp files are relative to the gyp file so they
don't require to have '../platform2/' in them.

BUG=None
TEST=cbuildbot amd64-generic-full arm-generic-full

Change-Id: I4da32969fc4956df14e9d3685f790565cd33d20f
Reviewed-on: https://chromium-review.googlesource.com/219788


Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Alex Deymo <deymo@chromium.org>
Tested-by: Alex Deymo <deymo@chromium.org>