Makefile.rules: Fix silencing of libec.a related warnings.
Makefile.rules: Fix silencing of libec.a related warnings.

When sanitizers are enabled, the libec.a related steps print a very
large amount of warnings that polute the build log and make it hard to
troubleshoot.

BRANCH=none
BUG=chromium:950310
TEST=USE="asan fuzzer" emerge-${BOARD} chromeos-ec

Change-Id: Ica06322291c95595b01941cffcfc297f2368ca02
Signed-off-by: Allen Webb <880184b418addffb87cd9c8e8e882ca6764ac96b@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/1566655
Reviewed-by: Andrey Pronin <53bbebe6910b11f7aa2d1daaa466658eb4cba5f2@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec