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_minix.c b/util-linux/volume_id/unused_minix.c
index 2f52093..4f4f8a7 100644
--- a/util-linux/volume_id/unused_minix.c
+++ b/util-linux/volume_id/unused_minix.c
@@ -31,7 +31,7 @@
 	uint16_t	s_magic;
 	uint16_t	s_state;
 	uint32_t	s_zones;
-} __attribute__((__packed__));
+} PACKED;
 
 #define MINIX_SUPERBLOCK_OFFSET			0x400