[Libguestfs] OCaml 5.0 warning

When running make with OCaml 5.0 installed, I get some warnings complaining about changed lib directory layout. It suggests adding "-I +unix -I +str" to the invocation of ocamlc. So this patch does just that. It does silence the warnings for me, but someone should probably test that it works with older versions of OCaml as well. See the commit message for more details. Best regards, Tage