Use function declaration for getdents64

Use function declaration for getdents64

When defining `getdents64` as a macro aliasing `getdents`, the
syscall shim layer complains.

This patch uses a typical C-style declaration of `getdents64()`. This,
combined with the `LFS64(getdents)`, aliasing used in
`src/dirent/__getdents.c` solves the issue with the syscall shim layer.

Signed-off-by: Razvan Deaconescu <razvand@unikraft.io>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Approved-by: Simon Kuenzer <simon@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #31