Fixes size and alignment computation of bitfields.

Tested on windows i386 with haskell-platform 2013.2.0.0.

Without this fix, the following struct would show different
sizes with c2hs {#sizeof bitfields #} and gcc sizeof(bitfields).

typedef struct {
    unsigned int b0  :  31;
    unsigned int b30  :  1;
} bitfields;
Merge branch 'bitfield-sizeAlign-fix' of https://github.com/facundominguez/c2hs into facundominguez-bitfield-sizeAlign-fix