fix(annotate): handle last input line without trailing newline

[zsbahtiar]

• Fix silent data loss when input file doesn't end with a trailing newline
• The last line was skipped because ReadString('\n') returns content alongside io.EOF, and the old loop only processed lines when err == nil

tested:

printf "1.1.1.1\n8.8.8.8" > /tmp/no_newline.txt
cat /tmp/no_newline.txt | go run cmd/zannotate/main.go --geoasn --geoasn-database=./data-snapshots/geolite2_asn.mmdb
{"geoasn":{"asn":13335,"org":"CLOUDFLARENET"},"ip":"1.1.1.1"}
{"geoasn":{"asn":15169,"org":"GOOGLE"},"ip":"8.8.8.8"}

solve: #57