use PACKED macro insted of open-coding GCC-ism

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/util-linux/volume_id/unused_mac.c b/util-linux/volume_id/unused_mac.c
index 8eaa173..c4732ee 100644
--- a/util-linux/volume_id/unused_mac.c
+++ b/util-linux/volume_id/unused_mac.c
@@ -24,7 +24,7 @@
 	uint8_t		signature[2];
 	uint16_t	block_size;
 	uint32_t	block_count;
-} __attribute__((__packed__));
+} PACKED;
 
 struct mac_partition {
 	uint8_t		signature[2];
@@ -34,7 +34,7 @@
 	uint32_t	block_count;
 	uint8_t		name[32];
 	uint8_t		type[32];
-} __attribute__((__packed__));
+} PACKED;
 
 int volume_id_probe_mac_partition_map(struct volume_id *id, uint64_t off)
 {