pstat(8): improve the Size header width after r358181

All size values use fields of width 8. Also, all other headers use fields of
width 8. Make the Size header added in r358181 use 8 characters as well.

usr.sbin/pstat/pstat.c

@@ -475,7 +475,7 @@ print_swap_header(void)
 
 	if (humanflag) {
 		header = SIZEHDR;
-		hlen = sizeof(SIZEHDR);
+		hlen = 8; /* as the hardcoded field width of values */
 	} else {
 		header = getbsize(&hlen, &blocksize);
 	}