v2msvc: Fix compilation error due to missing mktemp() declaration

> Commit d1b6e6e (win32: use our own dirent.h, 2010-11-23) removed
> the compat/vcbuild/include/dirent.h compatibility header file.
> This file, among other things, included the <io.h> system header
> file which provides the declaration of the mktemp() function.
> 
> In order to fix the compilation error, we add an include directive
> for <io.h> to the compat/mingw.h header.
> 
> Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
> ---
> 
> Change from v1:
>     - add #include to compat/mingw.h rather than compat/vcbuild/include/unistd.h
> 
> ATB,
> Ramsay Jones
> 
>  compat/mingw.h |    1 +
>  1 files changed, 1 insertions(+), 0 deletions(-)
> 
> diff --git a/compat/mingw.h b/compat/mingw.h
> index cafc1eb..1c6bc02 100644
> --- a/compat/mingw.h
> +++ b/compat/mingw.h
> @@ -1,5 +1,6 @@
>  #include <winsock2.h>
>  #include <ws2tcpip.h>
> +#include <io.h>
>  
>  /*
>   * things that are not available in header files