Fix the following build failure with glibc >= 2.34 raised because `closefrom` doesn't return an `int` but a `void` since its addition with https://github.com/bminor/glibc/commit/607449506f197cc9514408908f41f22537a47a8c
```
posix-io.c: In function '_gpgme_io_spawn':
posix-io.c:577:23: error: void value not ignored as it ought to be
577 | while ((i = closefrom (fd)) && errno == EINTR)
| ^
```