respect CXXFLAGS when building protobuf objs

We need to pass CXXFLAGS to the add_object_rules call.

BUG=None
TEST=`emerge-mipsel-o32-generic mtpd` passes now

Change-Id: Ia739e06d30592dc2135e1b57d7cb7d8bfbcc073f
Reviewed-on: https://chromium-review.googlesource.com/272737
Trybot-Ready: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Yuki Awano <yawano@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
1 file changed