dlopen_test.go:54:15: undefined: strlen

Go 1.15 beta 1 on Fedora Rawhide, during go testing:

```
github.com/coreos/go-systemd/internal/dlopen
# github.com/coreos/go-systemd/internal/dlopen [github.com/coreos/go-systemd/internal/dlopen.test]
./dlopen_test.go:54:15: undefined: strlen
FAIL    github.com/coreos/go-systemd/internal/dlopen [build failed]
```