filter_lua: add index to arraylength call

When building with `FLB_FILTER_LUA_USE_MPACK`, compilation fails because
this `flb_lua_arraylength` call didn't have an index argument. This PR
passes the index argument to the function that I believe to be correct,
which is `-1` (assuming based on its usage in the following loop to get
individual elements).

Signed-off-by: braydonk <[email protected]>

plugins/filter_lua/lua.c

@@ -173,7 +173,7 @@ static void pack_result_mpack(lua_State *l,
         return;
     }
 
-    len = flb_lua_arraylength(l);
+    len = flb_lua_arraylength(l, -1);
     if (len > 0) {
         /* record split */
         for (i = 1; i <= len; i++) {