Go bindings: Build fails due to empty generated file

I think because of dependency problems in the make file, the `generated.go` file can sometimes be empty or (arguably worse) out of date. In the case where it's empty, it fails with this error:

```
Compiling fdb
can't load package: package github.com/apple/foundationdb/bindings/go/src/fdb:
bindings/go/build/src/github.com/apple/foundationdb/bindings/go/src/fdb/generated.go:1:1: expected 'package', found 'EOF'
make: *** [/Volumes/HomeData/Users/alec/foundation/foundationdb/bindings/go/build/pkg/darwin_amd64/github.com/apple/foundationdb/bindings/go/src/fdb.a] Error 1
```

I believe this to be from the fact that copying over the files does not take the generated file as a dependency.