src: avoid use of _init

Xtables-addons's extensions will always be built as modules, so it is
safe to use __attribute__((constructor)).

extensions/libxt_CHAOS.c

@@ -106,8 +106,7 @@ static struct xtables_target chaos_tg_reg = {
 	.extra_opts    = chaos_tg_opts,
 };
 
-void _init(void);
-void _init(void)
+static __attribute__((constructor)) void chaos_tg_ldr(void)
 {
 	xtables_register_target(&chaos_tg_reg);
 }