On Win32, currently, rb_nativethread_cond_t is an incomplete type
because it's a typedef for `struct rb_thread_cond_struct`. That means
you can't actually allocate a rb_nativethread_cond_t unless you also
include THREAD_IMPL_H (since its defined in thread_win32.h)
(alternatively, including vm_core.h also works).

Move the definition of rb_thread_cond_struct into thread_native.h to
alleviate this.