fixed compiler warning.

The compiler warning is described below.

```
../nginx_ajp_module/ngx_http_ajp_handler.c:65:17: error: implicit conversion from enumeration type 'ngx_http_ajp_state_e' to different enumeration type
      'ngx_http_ajp_packet_state_e' [-Werror,-Wenum-conversion]
    a->pstate = ngx_http_ajp_st_init_state;
```