Fix type of android_thread_func typedef

Found the error with latest clang: https://android-build.corp.google.com/artifact/pending/P80448506/art-host-x86_64/latest/view/logs%2Fbuild_error.log

```
out/soong/.intermediates/system/core/libutils/libutils/linux_glibc_x86_64_static/obj/system/core/libutils/Threads.o system/core/libutils/Threads.cpp
system/core/libutils/Threads.cpp:149:21: error: cast from 'android_thread_func_t' (aka 'int (*)(void *)') to 'android_pthread_entry' (aka 'void *(*)(void *)') converts to incompatible function type [-Werror,-Wcast-function-type-mismatch]
  149 |                     (android_pthread_entry)entryFunction, userData);
      |                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
```

Change-Id: Ie2423a568483c975a36a1fa0b06a3f93a27be5aa