seq: use ATTRIBUTE_UNUSED instead remove argument name
We need to support older compilers than GCC 11.
Link: https://github.com/alsa-project/alsa-utils/issues/233
Fixes: 181e190 ("aplaymidi: fix the verbose compilation warnings for latest gcc")
Fixes: a03377a ("aseqnet: fix the verbose compilation warnings for latest gcc")
Fixes: 429c32a ("aseqdump: fix the verbose compilation warnings for latest gcc")
Fixes: 0b48dd6 ("aconnect: fix the verbose compilation warnings for latest gcc")
Signed-off-by: Jaroslav Kysela <perex@perex.cz>