metrics: 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: Ibe02d33d7d17a830f0d72cdb2597b8e97b1864da
Reviewed-on: https://chromium-review.googlesource.com/201982


Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>