[MSVC] offsetof implicit definition.

```C++
typedef struct _APC_CALL_FRAME
{
    void* Routine;
    unsigned long SizeOfStackFrame;

} APC_CALL_FRAME, *PAPC_CALL_FRAME;
C_ASSERT(offsetof(APC_CALL_FRAME, SizeOfStackFrame) == 0x8);
```

The above code does not compile unless cstddef is included.
MSVC defines offsetof and does not require including cstddef.