fixed a C compiler warning on FreeBSD introduced in the previous commit.

Author: agentzh

src/ngx_http_lua_socket_tcp.c

@@ -4946,8 +4946,7 @@ ngx_http_lua_socket_push_input_data(ngx_http_request_t *r,
 
 #if (NGX_DTRACE)
     ngx_http_lua_probe_socket_tcp_receive_done(r, u,
-                                               (const u_char *)
-                                               lua_tostring(L, -1),
+                                               (u_char *) lua_tostring(L, -1),
                                                size);
 #endif