pkg/system/dumpcapext.go build constraints are not specific enough

FreeBSD/arm64 build currently fails with

```
[00:04:26] # github.com/gcla/termshark/v2/pkg/system
[00:04:26] pkg/system/dumpcapext_arm64.go:24:6: DumpcapExt redeclared in this block
[00:04:26] 	pkg/system/dumpcapext.go:28:6: other declaration of DumpcapExt
```

due to the both `pkg/system/dumpcapext.go` and `pkg/system/dumpcapext_arm64.go` being included in the build.