use PACKED macro insted of open-coding GCC-ism

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

util-linux/volume_id/jfs.c

@@ -31,7 +31,7 @@ struct jfs_super_block {
 	uint8_t		uuid[16];
 	uint8_t		label[16];
 	uint8_t		loguuid[16];
-} __attribute__((__packed__));
+} PACKED;
 
 #define JFS_SUPERBLOCK_OFFSET			0x8000