[PATCH] [gdb/build] Use const_cast in fd_copy

[Eli Zaretskii]

> From: Tom de Vries <tdevries@suse.de>
> Date: Sat,  4 Jan 2025 11:54:15 +0100
> 
> Recent commit 6ab5d62ebc5 ("[gdb] Fix compilation error in event-top.c") did:
> ...
>  fd_copy (fd_set *dst, const fd_set *src, int n)
>  {
>    FD_ZERO (dst);
>    for (int i = 0; i < n; ++i)
> -    if (FD_ISSET (i, src))
> +    if (FD_ISSET (i, (fd_set *)src))
> ...
> but according to [1] only const_cast may be used to cast away constness.
> 
> Fix this by using const_cast.
> 
> Tested by rebuilding on x86_64-linux.

Works for me, thanks.

Please also cherry-pick the fix to gdb-16-branch.