Changed example to use a C function available also on Windows

[vikz95]

getuid() is a clib function that's only available on UNIX systems, this pull request uses the time() function that's also available on Windows.

Now when running cargo test on Windows the command doesn't fail with error message:

error[E0601]: `main` function not found in crate `ffi`
  --> mockall\examples\ffi.rs:35:2
   |
35 | }
   |  ^ consider adding a `main` function to `mockall\examples\ffi.rs`