use PACKED macro insted of open-coding GCC-ism

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

util-linux/volume_id/unused_via_raid.c

@@ -29,10 +29,10 @@ struct via_meta {
 		uint32_t	capacity_low;
 		uint32_t	capacity_high;
 		uint32_t	serial_checksum;
-	} __attribute((packed)) array;
+	} PACKED array;
 	uint32_t	serial_checksum[8];
 	uint8_t		checksum;
-} __attribute__((packed));
+} PACKED;
 
 #define VIA_SIGNATURE		0xAA55