[aarch64] Missing `compat.h` causes build failure

We need to include `compat.h` for aarch64 builds, but that file is missing from the Nimble codebase.
```C++
#ifdef __x86_64__
#include <immintrin.h>
#endif //__x86_64__

#ifdef __aarch64__
#include "common/aarch64/compat.h"
#endif //__aarch64__
```
https://github.com/facebookincubator/nimble/blob/main/dwio/nimble/common/Varint.cpp#L21

cc @xiaoxmeng @Yuhta 
