timespec_sub bug fix

[Bruno Haible]

I needed more infos about this bug fix (so I can know why the CI did
not report it within three weeks). Since bug-gnulib was not in CC, I had
to search. I updated this ChangeLog entry:


2024-07-10  Pip Cet  <pip...@protonmail.com>

        timespec-sub: Fix compilation error on clang.
        Reported by Gerd Möllmann <gerd.moellm...@gmail.com> at
        <https://debbugs.gnu.org/cgi/bugreport.cgi?bug=72036>.
        * lib/timespec-sub.c (timespec_sub): Use 'int' as type of variable.
        Copyright-paperwork-exempt: Yes