ASoC: uapi: Replace zero-length arrays with __DECLARE_FLEX_ARRAY() helper

Zero-length arrays are deprecated and we are moving towards adopting
C99 flexible-array members, instead. So, replace zero-length arrays
declarations in anonymous union with the new __DECLARE_FLEX_ARRAY()
helper macro.

This helper allows for flexible-array members in unions.

Link: https://github.com/KSPP/linux/issues/193
Link: https://github.com/KSPP/linux/issues/227
Link: https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
Reviewed-by: Kees Cook <[e-mail]>
Reviewed-by: Amadeusz Sławiński <[e-mail]>
Signed-off-by: Gustavo A. R. Silva <[e-mail]>

include/uapi/sound/asoc.h

···
 	__le32 type;	/* SND_SOC_TPLG_TUPLE_TYPE_ */
 	__le32 num_elems;	/* number of elements in array */
 	union {
-		struct snd_soc_tplg_vendor_uuid_elem uuid[0];
-		struct snd_soc_tplg_vendor_value_elem value[0];
-		struct snd_soc_tplg_vendor_string_elem string[0];
+		__DECLARE_FLEX_ARRAY(struct snd_soc_tplg_vendor_uuid_elem, uuid);
+		__DECLARE_FLEX_ARRAY(struct snd_soc_tplg_vendor_value_elem, value);
+		__DECLARE_FLEX_ARRAY(struct snd_soc_tplg_vendor_string_elem, string);
 	};
 } __attribute__((packed));