selftests/alsa: make dump_config_tree() as void function

dump_config_tree() is declared to return an int, but the compiler cannot
prove that it always returns any value at all. This leads to a clang
warning, when building via:

make LLVM=1 -C tools/testing/selftests

Suggested-by: John Hubbard <[e-mail]>
Cc: Mark Brown <[e-mail]>
Signed-off-by: Jaroslav Kysela <[e-mail]>
Reviewed-by: Mark Brown <[e-mail]>
Signed-off-by: Takashi Iwai <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

tools/testing/selftests/alsa/conf.c

···
 	return NULL;
 }
 
-static int dump_config_tree(snd_config_t *top)
+static void dump_config_tree(snd_config_t *top)
 {
 	snd_output_t *out;
 	int err;