Fix [-Wundef] warning in Date extension

Author: Girgias

ext/date/lib/timelib_private.h

@@ -49,15 +49,15 @@
 # include <stdint.h>
 #endif
 
-#if HAVE_UNISTD_H
+#ifdef HAVE_UNISTD_H
 # include <unistd.h>
 #endif
 
-#if HAVE_IO_H
+#ifdef HAVE_IO_H
 # include <io.h>
 #endif
 
-#if HAVE_DIRENT_H
+#ifdef HAVE_DIRENT_H
 # include <dirent.h>
 #endif