ALSA: core: Replace zero-length array with flexible-array member

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertenly introduced[3] to the codebase from now on.

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732 ("cxgb3/l2t: Fix undefined behaviour")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Link: https://lore.kernel.org/r/20200211193910.GA4596@embeddedorSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/oss/rate.c

@@ -47,7 +47,7 @@ struct rate_priv {
 	unsigned int pos;
 	rate_f func;
 	snd_pcm_sframes_t old_src_frames, old_dst_frames;
-	struct rate_channel channels[0];
+	struct rate_channel channels[];
 };
 
 static void rate_init(struct snd_pcm_plugin *plugin)