lua: add flex array field to TString type

Linux 6.10+ with CONFIG_FORTIFY_SOURCE notices memcpy() accessing past
the end of TString, because it has no indication that there there may be
an additional allocation there.

There's no appropriate upstream change for this (ancient) version of
Lua, so this is the narrowest change I could come up with to add a flex
array field to the end of TString to satisfy the check. It's loosely
based on changes from lua/lua@ca41b43f and lua/lua@9514abc2.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #16541
Closes #16583