Set proper `CTYPE` for POSIX `CGroup`

This fixes https://github.com/haskell/unix/issues/20 which is about
compile warnings due to incompatible pointer types of the kind

     warning: passing argument 5 of ‘getgrnam_r’ from incompatible pointer type
     HsInt32 ghc...(void* a1, void* a2, void* a3, HsWord64 a4, void** a5)
       {return getgrnam_r(a1, a2, a3, a4, a5);}

     note: expected ‘struct group ** __restrict__’ but argument is of type ‘void **’
     extern int getgrnam_r (const char *__restrict __name,

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>