Fix warning unused parameter 'reply_timestamp' under macOS

[gsnw-sebast]

Under macOS the following build warning is fixed

gcc -DHAVE_CONFIG_H -I. -I..    -Wall -Wextra -Wno-sign-compare -DIPV6 -g -O0 -fprofile-arcs -ftest-coverage -MT fping-fping.o -MD -MP -MF .deps/fping-fping.Tpo -c -o fping-fping.o `test -f 'fping.c' || echo './'`fping.c
fping.c:2027:14: warning: unused parameter 'reply_timestamp' [-Wunused-parameter]
    int64_t *reply_timestamp,
             ^
1 warning generated.

[coveralls]

coverage: 87.526% (-0.2%) from 87.746%
when pulling 9cef0be on gsnw:macos-reply_timestamp
into c0eb514 on schweikert:develop.

[auerswal]

Thanks, this looks good. :-)

The changes look plausible. Without these changes, the macOS build on the GitHub CI reports the warning. With them, it does not.