don't warn on *const c_void -> *const T casts

[oli-obk]

We can detect the c_void type via its path (it's defined in https://docs.rs/libc/0.2.40/src/libc/lib.rs.html#115-121 )

The user has to take care around c_void casts anyway.

cc @devonhollowood

[ivanovaleksey]

Hello, seem like I came across this issue too while running clippy on Paho MQTT.
https://github.com/eclipse/paho.mqtt.rust/blob/master/src/async_client.rs#L377

error: casting from `*mut std::os::raw::c_void` to a less-strictly-aligned pointer (`*mut async_client::AsyncClient`)
   --> vendor/paho.mqtt.rust/src/async_client.rs:377:14
    |
377 |             let cli = context as *mut AsyncClient;
    |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.194/index.html#cast_ptr_alignment

But it seems to me that it's the only possible way to work with raw pointers, so, it's legal operation.

[YaLTeR]

Yep, came across this in a FFI wrapper crate I'm working on as well.