sound tests: Fix gcc build

/workspace/src/tests/sys/sound/pcm_read_write.c:36:1: error: 'static' is not
at beginning of declaration [-Werror=old-style-declaration]

36 | } static const afmt_tests[] = {
   | ^

Reported by: CI
Fixes: 27ef5d48c729 ("sound: Unit test the pcm sample read and write macros")
Sponsored by: The FreeBSD Foundation
MFC after: 1 week