libassuan build failure on macOS 15.1.1

[NIIBE Yutaka]

"Robert J. Hansen" <rjh at sixdemonbag.org> wrote:
> rjh at sarah libassuan-3.0.1 % grep -r ___sputc | grep -v "Binary file"
> rjh at sarah libassuan-3.0.1 %
>
> There is no such declaration.

On macOS, __sputc is an inline function in stdio.h, and it is used by
the macro of putc_unlocked.  Removing unused putc_unlocked declaration
in libassuan fixed the problem.
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