rust: Do not use deprecated integer types

rust: Do not use deprecated integer types

In order to avoid warning like:

warning: use of deprecated item 'libc::uint64_t': Use u64 instead.
  --> src/lib.rs:42:19
   |
42 |     _struct_size: uint64_t,
   |                   ^^^^^^^^
   |
   = note: #[warn(deprecated)] on by default

and

warning: use of deprecated item 'libc::fixed_width_ints::int64_t': Use i64 instead.
  --> examples/ramdisk.rs:68:49
   |
68 | extern fn ramdisk_get_size (_h: *mut c_void) -> int64_t {
   |                                                 ^^^^^^^
   |
   = note: #[warn(deprecated)] on by default

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
Message-Id: <debcc47784c35265dcfd12d869ec9aead9a7f99d.1560252047.git.mkletzan@redhat.com>