_ggml_backend_blas_reg missing on darwin/arm build

When building on darwin / aarch64:

```
[ 55%] Linking CXX shared library librwkv.dylib
Undefined symbols for architecture arm64:
  "_ggml_backend_blas_reg", referenced from:
      ggml_backend_registry::ggml_backend_registry() in ggml-backend-reg.cpp.o
ld: symbol(s) not found for architecture arm64
```