declare struct tftphdr and embedded union as beeing packed, which is

required for arm.

Tested on:	arm, i386, amd64
Approved by:	re (rwatson)
MFC after:	1 week

include/arpa/tftp.h

@@ -58,9 +58,9 @@ struct tftphdr {
 		unsigned short	tu_block;	/* block # */
 		unsigned short	tu_code;	/* error code */
 		char	tu_stuff[1];	/* request packet stuff */
-	} th_u;
+	} __packed th_u;
 	char	th_data[1];		/* data or error string */
-};
+} __packed;
 
 #define	th_block	th_u.tu_block
 #define	th_code		th_u.tu_code