busctl: fix assertion failure on --size option

busctl: fix assertion failure on --size option

Using --size option triggers an assert failure below because
parse_size() requires the second argument, base, being either 1000 or
1024.  As it's for a packet size, it'd be better using IEC binary
suffix (base 1024) IMHO.

  $ busctl --size 2048
  Assertion 'base == 1000 || base == 1024' failed at src/basic/util.c:2222,
  function parse_size(). Aborting.
  Aborted (core dumped)