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/linux_raid.c b/util-linux/volume_id/linux_raid.c
index cc02469..b691f20 100644
--- a/util-linux/volume_id/linux_raid.c
+++ b/util-linux/volume_id/linux_raid.c
@@ -37,7 +37,7 @@
 	uint32_t	set_uuid1;
 	uint32_t	set_uuid2;
 	uint32_t	set_uuid3;
-} __attribute__((packed));
+} PACKED;
 
 #define MD_RESERVED_BYTES		0x10000
 #define MD_MAGIC			0xa92b4efc