Cannot add rules with multiple key filters

Hello! Thanks for this great project!

I've encountered a problem: parsed rules with multiple key filters (`-F key=`) cannot be added to the kernel - an "invalid argument" error is returned.

I use the library like this:

```go
ruleWithTwoKeyFormats := "-a always,exit -F arch=b64 -S open,truncate,ftruncate,creat,openat,open_by_handle_at -F exit=-EACCES -F key=firstkey -F key=secondkey"

r, err := flags.Parse(ruleWithTwoKeyFormats)
if err != nil {
return err
}

data, err := rule.Build(r)
if err != nil {
return err
}

if err := client.AddRule(data); err != nil {
return err
}

fmt.Println("added rule")
```

When `-k ` syntax is used instead of `-F key=`, it works correctly. Mixing both formats again leads to this error.