Fix i_threads.c compiling on macOS

src/i_threads.c

@@ -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;