cmake : link threads publicly to ggml (#1042)

cmake : link threads publicly to ggml (#1042)

* fix: ld link test-tokenizer-0 error

```
cmake3 --build . --config Release
[  5%] Built target ggml
[ 16%] Built target llama
[ 22%] Linking CXX executable ../bin/test-tokenizer-0
../libllama.a(ggml.c.o)：在函数‘ggml_graph_compute’中：
ggml.c:(.text+0xf2db)：对‘pthread_create’未定义的引用
ggml.c:(.text+0xf9d4)：对‘pthread_join’未定义的引用
collect2: error: ld returned 1 exit status
gmake[2]: *** [bin/test-tokenizer-0] 错误 1
gmake[1]: *** [tests/CMakeFiles/test-tokenizer-0.dir/all] 错误 2
gmake: *** [all] 错误 2
```

* Update CMakeLists.txt

* Update CMakeLists.txt

* Update CMakeLists.txt