libassuan build failure on macOS 15.1.1

[Robert J. Hansen]

> The fix is simply remove the (unused) declaration in assuan-defs.h.

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.

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