BUILD: lua: silence a build warning with TCC

BUILD: lua: silence a build warning with TCC

TCC doesn't have the equivalent of __builtin_unreachable() and complains
that hlua_panic_ljmp() may return no value. Let's add a return 0 there.
All compilers that know that longjmp() doesn't return will see no change
and tcc will be happy.

(cherry picked from commit 6a510907807b7fb901654b4f5e5100aa91868fb7)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 69c66e3584ac81b70dd445ff56f8c9815de8f28c)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>