[libctf] Fix warning: @xref should not appear on @multitable line

[libctf] Fix warning: @xref should not appear on @multitable line

When building gdb, I run into:
...
ctf-spec.texi:809: warning: @xref should not appear on @multitable line
...

The line in question is:
...
@multitable {Kind} {@code{CTF_K_VOLATILE}} {Indicates a type that cannot be represented in CTF, or that} {@xref{Pointers typedefs and cvr-quals}}
...
which defines a prototype row with 4 columns.

However, the table only has 3 colums:
...
@headitem Kind @tab Macro @tab Purpose
...

Fix the warning by removing the item in the prototype row representing a fourth column.

Tested on aarch64-linux.

PR libctf/32044
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32044