QDateTime: fix build with MinGW: need unistd.h very early

qobject.h has #include <chrono>, which #includes <ctime>, which includes
<time.h>.

datetime.cpp:2621:23: error: 'localtime_r' was not declared in this scope; did you mean 'localtime_s'?

Pick-to: 6.2
Change-Id: I2bbf422288924c198645fffd16a922719c4ce7d4
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

mkspecs/win32-g++/qplatformdefs.h

@@ -46,6 +46,8 @@
 #endif
 #endif
 
+#include <unistd.h> // Defines _POSIX_THREAD_SAFE_FUNCTIONS and others
+
 // Get Qt defines/settings
 
 #include "qglobal.h"

src/corelib/time/qdatetime.cpp

@@ -66,9 +66,6 @@
 #  include <qt_windows.h>
 #endif
 #include <time.h>
-#ifdef Q_CC_MINGW
-#  include <unistd.h> // Define _POSIX_THREAD_SAFE_FUNCTIONS to obtain localtime_r()
-#endif
 
 QT_BEGIN_NAMESPACE