crash-reporter: Update gyp file to use src/platform2/common-mk

We are moving the packages built by platform2's ebuild into src/platform2.
We need packages to use src/platform2/common-mk instead of
src/platform/common-mk.

BUG=chromium:378554
TEST=emerge-daisy platform2
TEST=trybot run on daisy, link, mario and duck

CQ-DEPEND=CL:201938

Change-Id: Ic154c59f61f7f95df5678a2033b74983b12f4dec
Reviewed-on: https://chromium-review.googlesource.com/201967


Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>