Move rb_thread_cond_struct definition into thread_native.h

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.

include/ruby/thread_native.h

@@ -28,6 +28,11 @@ typedef union rb_thread_lock_union {
     CRITICAL_SECTION crit;
 } rb_nativethread_lock_t;
 
+struct rb_thread_cond_struct {
+    struct cond_event_entry *next;
+    struct cond_event_entry *prev;
+};
+
 typedef struct rb_thread_cond_struct rb_nativethread_cond_t;
 
 #elif defined(HAVE_PTHREAD_H)

thread_win32.h

@@ -21,11 +21,6 @@
 WINBASEAPI BOOL WINAPI
 TryEnterCriticalSection(IN OUT LPCRITICAL_SECTION lpCriticalSection);
 
-struct rb_thread_cond_struct {
-    struct cond_event_entry *next;
-    struct cond_event_entry *prev;
-};
-
 struct rb_native_thread {
     HANDLE thread_id;
     HANDLE interrupt_event;