Add unix dependency to xenctrl

Without this, compiling things based on xenctrl with obuild can fail with errors like this:

```
File "_none_", line 1:
Error: No implementations provided for the following modules:
         Unix referenced from /usr/local2/.opam/4.00.1+annot/lib/xenctrl/xenctrl.cmxa(Xenctrl)

make: *** [build] Error 7
```

I was worried this might just be working around a bug in obuild, but the same thing happens with ocamlopt (note the order of the library files):

```
root@localhost:test [master]$ ocamlopt -I /usr/local2/.opam/4.00.1+annot/lib/xenctrl/ -I /usr/local2/.opam/4.00.1+annot/lib/ocaml xenctrl.cmxa unix.cmxa memory_summary.ml 
File "memory_summary.ml", line 1:
Error: No implementations provided for the following modules:
         Unix referenced from /usr/local2/.opam/4.00.1+annot/lib/xenctrl/xenctrl.cmxa(Xenctrl)
root@localhost:test [master]$ ocamlopt -I /usr/local2/.opam/4.00.1+annot/lib/xenctrl/ -I /usr/local2/.opam/4.00.1+annot/lib/ocaml unix.cmxa xenctrl.cmxa memory_summary.ml 
root@localhost:test [master]$
```

Compiling