w32-posix: Wrap the `timezone` declaration with a clause

Allows compilation in newer versions of MinGW that already defined it.

src/win32/posix_w32.c

@@ -487,11 +487,14 @@ p_gmtime_r (const time_t *timer, struct tm *result)
 #define DELTA_EPOCH_IN_MICROSECS  11644473600000000ULL
 #endif
  
+#ifndef _TIMEZONE_DEFINED
+#define _TIMEZONE_DEFINED
 struct timezone 
 {
    int  tz_minuteswest; /* minutes W of Greenwich */
    int  tz_dsttime;     /* type of dst correction */
 };
+#endif
  
 int p_gettimeofday(struct timeval *tv, struct timezone *tz)
 {