Fail to build on sparc64: mismatched types for TvUsecType

[jonglezb]

I'm evaluating the portability status of an application that is using libpnet. It turns out libpnet doesn't build on sparc64:

error[E0308]: mismatched types
  --> pnet_sys/src/unix.rs:97:22
   |
97 |             tv_usec: dur.subsec_micros() as TvUsecType,
   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `i32`, found `i64`

For more information about this error, try `rustc --explain E0308`.
error: could not compile `pnet_sys` due to previous error

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