size and offset errors with bit-fields and typedefs

This happens on windows with 32-bit version of ghc-7.10.3, c2hs-0.28.1, gcc-5.2.0.

gcc and c2hs disagree on the size of the `bitfields0` struct below.

```
// bitfields.h
typedef unsigned int an_int;
typedef struct {
an_int b0 : 5;
unsigned int b1 : 27;
} bitfields0;

typedef struct {
bitfields0 a;
unsigned int* p0;
unsigned int* p1;
unsigned int* p2;
} bitfields;
```

C test program

```
// bitfields.c
#include "bitfields.h"
#include

int main(int argc, char** argv) {
bitfields b;
printf("size of bitfields: %lu\n",sizeof(bitfields));
printf("offset of p1: %d\n"
, (int)(((void*)&b.p1) - (void*)&b));
return 0;
}
```

Output:

```
$ ./a.exe
size of bitfields: 16
offset of p1: 8
```

c2hs program

```
-- Bitfields.chs
module Bitfields where

import Foreign.C.Types
#include "bitfields.h"

s :: Int
s = {#sizeof bitfields #}

o :: Ptr () -> IO CUInt
o = {#get bitfields->p1 #}
```

Output:

```
module Bitfields where
import qualified Foreign.C.Types as C2HSImp
import qualified Foreign.Ptr as C2HSImp
import qualified Foreign.Storable as C2HSImp

import Foreign.C.Types

s :: Int
s = 20

o :: Ptr () -> IO CUInt
o = (\ptr -> do {C2HSImp.peekByteOff ptr 12 :: IO (C2HSImp.Ptr C2HSImp.CUInt)})
```