common.mk: update to _FORTIFY_SOURCE=3

Since this project explicitly specifies its FORTIFY level, update it to
the level CrOS wants to move to. This isn't _removed_, since it's copied
from a template file, and consistency with that seems useful.

The template file this was copied from was updated in
crrev.com/c/4735413.

BUG=b:288411201
TEST=CQ+1

Change-Id: I90d68c530e1d1f8faaaf42b1a1f78f4e7c74eeac
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/aver-updater/+/4800850
Reviewed-by: Kyle Williams <kdgwill@chromium.org>
Tested-by: George Burgess <gbiv@chromium.org>
Commit-Queue: George Burgess <gbiv@chromium.org>

• common.mk[diff]

$ make

$ emerge-${BOARD} aver-updater