Only define struct timespec if _TIMESPEC_DEFINED isn't set

This matches what the mingw-w64 headers define with the struct.

include/threads.h

@@ -33,11 +33,17 @@ typedef void (*altss_dtor_t)(void*);
 #include <windows.h>
 
 
-#ifndef __MINGW32__
+#ifndef _TIMESPEC_DEFINED
+#define _TIMESPEC_DEFINED
 struct timespec {
     time_t tv_sec;
     long tv_nsec;
 };
+
+struct itimerspec {
+    struct timespec it_interval;
+    struct timespec it_value;
+};
 #endif
 
 typedef DWORD althrd_t;