mount_gpt_image.sh: Fix path to include chromeos-common.sh

mount_gpt_image.sh can be run outside of the chroot. In that case it
will include file from the source code, not the installed location.
Reflect the code change location.

BUG=b:72749926
TEST=tryjob novato.

Change-Id: I6ca9d97ec1c3db5d91f048f459aa045a41437ab3
Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1042852
Tested-by: Gwendal Grignou <gwendal@google.com>
Trybot-Ready: Gwendal Grignou <gwendal@google.com>
Reviewed-by: Nicolas Norvez <norvez@chromium.org>
Commit-Queue: Hung-Te Lin <hungte@chromium.org>
1 file changed