bug#78701: Undeclared reference to access on Windows

[Kirill Makurin]

Building libltdl on Windows using clang-cl.exe fails with the following diagnostic:

```

H:/git/libtool/libltdl/ltdl.c(781,19): error: call to undeclared function 'access'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]

  781 |   int           notfound        = access (filename, R_OK);

      |                                   ^

H:/git/libtool/libltdl/ltdl.c(781,19): note: did you mean 'accept'?

C:\Program Files (x86)\Windows Kits\10\\include\10.0.22000.0\\um\winsock.h(739,19): note: 'accept' declared here

  739 | SOCKET PASCAL FAR accept (

      |                   ^

```

On native Windows (MSVC/mingw), access is declared in io.h, which is not included. (On mingw, it could be indirectly included from

other header files.)

A solution would be to add 

```

#if defined (_WIN32) && !defined (__CYGWIN__)

#include <io.h>

#endif

```

on top of libltdl/ltdl.c.

- Kirill Makurin