win32: _mingw.h: do not undef NULL

Not wise if stddef.h was already included. This is related to commit

    3aa26a794e

Instead hack stddef.h to have identical definition and thus
avoid the issue mentionned there.

include/stddef.h

@@ -20,7 +20,7 @@ typedef unsigned int uint32_t;
 typedef unsigned long long int uint64_t;
 #endif
 
-#define NULL ((void *)0)
+#define NULL ((void*)0)
 #define offsetof(type, field) ((size_t)&((type *)0)->field)
 
 void *alloca(size_t size);