gwin32.c: Fix build on MinGW

Apparently unlike mingw-w64 and Visual Studio, MinGW does not come with
winternl.h, which defines NTSTATUS, so we need to include ntdef.h instead
on MinGW for NTSTATUS.

Based on patch by Cédric Krier.

https://bugzilla.gnome.org/show_bug.cgi?id=756875

glib/gwin32.c

@@ -59,6 +59,9 @@
 #ifdef _MSC_VER
 #pragma comment (lib, "ntoskrnl.lib")
 #endif
+#elif defined (__MINGW32__)
+/* mingw-w64, not MinGW, has winternl.h */
+#include <ntdef.h>
 #else
 #include <winternl.h>
 #endif