openbsd: Fix some clippy warnings to use pointer::cast.

[collinfunk]

This change just fixes some clippy warnings that are found on OpenBSD like:

warning: `as` casting between raw pointers without changing their constness
   --> src/unix/bsd/netbsdlike/openbsd/mod.rs:633:11
    |
633 |         (*(self as *const siginfo_t as *const siginfo_timer))._pid
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try `pointer::cast`, a safer alternative: `(self as *const siginfo_t).cast::<siginfo_timer>()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_as_ptr
    = note: requested on the command line with `-W clippy::ptr-as-ptr`

@rustbot label +stable-nominated

[rustbot]

Some changes occurred in OpenBSD module

cc @semarie

[collinfunk]

Oops, s/net/open. Let me ammend the commit message and force push...

[tgross35]

Thanks! I'll need to wait for #4489 before merging this.