MinGW: Fix developer build

In some build configurations, the build with MinGW would fail due to missing
declaration of localtime_r(). This is only visible when
_POSIX_THREAD_SAFE_FUNCTIONS is defined, which is achieved by including
<unistd.h>. Amends 6c543879a3.

Task-number: QTBUG-71030
Change-Id: I71296f24a450159d1c548e1ad836a2b42e42009f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qglobal_p.h

@@ -61,6 +61,9 @@
 #endif
 
 #if defined(__cplusplus)
+#ifdef Q_CC_MINGW
+#  include <unistd.h> // Define _POSIX_THREAD_SAFE_FUNCTIONS to obtain localtime_r()
+#endif
 #include <time.h>
 
 QT_BEGIN_NAMESPACE

src/corelib/tools/qdatetime.cpp

@@ -58,6 +58,9 @@
 #endif
 
 #include <cmath>
+#ifdef Q_CC_MINGW
+#  include <unistd.h> // Define _POSIX_THREAD_SAFE_FUNCTIONS to obtain localtime_r()
+#endif
 #include <time.h>
 #ifdef Q_OS_WIN
 #  include <qt_windows.h>