Don't scan libfoo_error.c for error codes.

This will have the effect that all dead error codes are removed
from given lib when make_errors.go runs with --reset flag.

Change-Id: I6303721c5d7cd18af7d47c95fdf3702a7628ad5a
Reviewed-on: https://boringssl-review.googlesource.com/1570
Reviewed-by: Adam Langley <agl@google.com>