New build warnings on Windows

[Bruce Cran <bruce@cran.org.uk>]

There are some new warnings in 2.1.5 when building on Windows:

In file included from io_u.c:14:0:
err.h: In function �ERR_PTR�:
err.h:18:9: warning: cast to pointer from integer of different size 
[-Wint-to-pointer-cast]
return (void *) error;
^
err.h: In function �PTR_ERR�:
err.h:23:9: warning: cast from pointer to integer of different size 
[-Wpointer-to-int-cast]
return (long) ptr;
^
err.h: In function �IS_ERR�:
err.h:28:22: warning: cast from pointer to integer of different size 
[-Wpointer-to-int-cast]
return IS_ERR_VALUE((unsigned long)ptr);
^
err.h:14:27: note: in definition of macro �IS_ERR_VALUE�
#define IS_ERR_VALUE(x) ((x) >= (unsigned long)-MAX_ERRNO)
^
err.h: In function �IS_ERR_OR_NULL�:
err.h:33:30: warning: cast from pointer to integer of different size 
[-Wpointer-to-int-cast]
return !ptr || IS_ERR_VALUE((unsigned long)ptr);
^
err.h:14:27: note: in definition of macro �IS_ERR_VALUE�
#define IS_ERR_VALUE(x) ((x) >= (unsigned long)-MAX_ERRNO)

-- 
Bruce