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/udf.c b/util-linux/volume_id/udf.c
index e272e19..a6e719a 100644
--- a/util-linux/volume_id/udf.c
+++ b/util-linux/volume_id/udf.c
@@ -30,28 +30,28 @@
 		uint16_t	crc;
 		uint16_t	crc_len;
 		uint32_t	location;
-	} __attribute__((__packed__)) tag;
+	} PACKED tag;
 	union {
 		struct anchor_descriptor {
 			uint32_t	length;
 			uint32_t	location;
-		} __attribute__((__packed__)) anchor;
+		} PACKED anchor;
 		struct primary_descriptor {
 			uint32_t	seq_num;
 			uint32_t	desc_num;
 			struct dstring {
 				uint8_t	clen;
 				uint8_t	c[31];
-			} __attribute__((__packed__)) ident;
-		} __attribute__((__packed__)) primary;
-	} __attribute__((__packed__)) type;
-} __attribute__((__packed__));
+			} PACKED ident;
+		} PACKED primary;
+	} PACKED type;
+} PACKED;
 
 struct volume_structure_descriptor {
 	uint8_t		type;
 	uint8_t		id[5];
 	uint8_t		version;
-} __attribute__((__packed__));
+} PACKED;
 
 #define UDF_VSD_OFFSET			0x8000