std: Fix Thread::set_name() for newlib and solaris

The `use ffi::CStr` in `unix/thread.rs` was previously guarded, but now
all platforms need it for `Thread::set_name()`.  Newlib and Solaris do
nothing here, as they have no way to set a thread name, but they still
define the same method signature.

src/libstd/sys/unix/thread.rs

@@ -12,7 +12,6 @@ use prelude::v1::*;
 
 use alloc::boxed::FnBox;
 use cmp;
-#[cfg(not(any(target_env = "newlib", target_os = "solaris")))]
 use ffi::CStr;
 use io;
 use libc;