libtorrent recipe build fail

### Logs

In addition to this error there are a lot of depreciated warnings - an accident waiting to happen.

```
/home/bobf/ex/regression/.buildozer/android/platform/build-arm64-v8a/build/other_builds/boost/arm64-v8a__ndk_target_21/boost/libs/python/src/exec.cpp:109:14: error: use of undeclared identifier '_Py_fopen'
FILE *fs = _Py_fopen(f, "r");
^
1 warning and 1 error generated.

```

Start by reproducing the libtorrent recipe build and tracing the failure in boost/libs/python/src/exec.cpp, where _Py_fopen is reported as undeclared. Done means the recipe builds without the shown compiler error; the reported deprecated warnings may also need separate assessment.

Written by the indexing model from the issue text.