mkrootfs.sh: add flags for dlsym

While running the example hello.aci, I get the error:
```
stage1/usr/bin/systemd-nspawn: symbol lookup error: stage1/fakesdboot.so: undefined symbol: dlsym
```

I'm not sure it is the best thing to do, but setting the flags `-Wl,--no-as-needed` solved my problem.

My configuration:
> Linux ubuntu 3.16.0-25-generic
> gcc version 4.9.1 (Ubuntu 4.9.1-16ubuntu6)