Fix i_threads.c compiling on macOS

It doesn't compile without including i_threads.h, even though macOS does not have an implementation for threads in this version.

SRB2Classic/src/i_threads.c:316:39: error: unknown type name 'I_thread_fn'
  316 | void I_spawn_thread(const char *name, I_thread_fn entry, void *userdata)
      |                                       ^
SRB2Classic/src/i_threads.c:334:19: error: unknown type name 'I_mutex'
  334 | void I_lock_mutex(I_mutex *anchor)
      |                   ^
SRB2Classic/src/i_threads.c:339:21: error: unknown type name 'I_mutex'
  339 | void I_unlock_mutex(I_mutex id)
      |                     ^
SRB2Classic/src/i_threads.c:344:18: error: unknown type name 'I_cond'
  344 | void I_hold_cond(I_cond *cond_anchor, I_mutex mutex_id)
      |                  ^
SRB2Classic/src/i_threads.c:344:39: error: unknown type name 'I_mutex'
  344 | void I_hold_cond(I_cond *cond_anchor, I_mutex mutex_id)
      |                                       ^
SRB2Classic/src/i_threads.c:350:22: error: unknown type name 'I_cond'
  350 | void I_wake_one_cond(I_cond *anchor)
      |                      ^
SRB2Classic/src/i_threads.c:355:22: error: unknown type name 'I_cond'
  355 | void I_wake_all_cond(I_cond *anchor)
      |                      ^

src/i_threads.c

@@ -9,7 +9,7 @@
 /// \file  i_threads.c
 /// \brief Multithreading abstraction
 
-#if defined (__unix__) || (!defined(__APPLE__) && defined (UNIXCOMMON))
+#if defined (__unix__) || defined(__APPLE__)
 
 #include <pthread.h>
 
@@ -313,6 +313,8 @@ void I_wake_all_cond(I_cond *anchor)
 	WakeAllConditionVariable(*anchor);
 }
 #else
+#include "i_threads.h"
+
 void I_spawn_thread(const char *name, I_thread_fn entry, void *userdata)
 {
 	(void)name;