gdb_bthread_stack.py not work.

**Describe the bug (描述bug)**
使用gdb查看bthread报错

```
(gdb) source /home/user/brpc/tools/gdb_bthread_stack.py
(gdb) bthread_begin
Active bthreads: 3, will display 3 bthreads
Python Exception No type "ResourcePool" within class or namespace "butil".:
Error occurred in Python: No type "ResourcePool" within class or namespace "butil".
(gdb) bthread_num
3
```

**To Reproduce (复现方法)**
编译example下的echo_c++/echo_server
执行echo_server
sudo gdb -p {pid}
然后如上面代码所示，加载gdb_bthread_stack.py，执行bthread_begin

**Expected behavior (期望行为)**
能够进入bthread debug mode

**Versions (各种版本)**
OS: ubuntu 20.04
Compiler: clang8
brpc: 1.8.0
protobuf: 3.15.8

**Additional context/screenshots (更多上下文/截图)**

Start with tools/gdb_bthread_stack.py and reproduce the reported failure by running echo_c++/echo_server, attaching gdb, sourcing the script, and executing bthread_begin. Investigate the missing butil::ResourcePool type reported by gdb. Done means bthread_begin enters bthread debug mode without the Python exception.

Written by the indexing model from the issue text.