fix compiler warning about variable shadowing
fix compiler warning about variable shadowing

Summary:
I started to see this warning today
```
fbandroid/native/redex/opt/interdex/CrossDexRelocator.cpp:96:10: warning: declaration shadows a local variable [-Wshadow]
    auto m = mref->as_def();
         ^
fbandroid/native/redex/opt/interdex/CrossDexRelocator.cpp:64:16: note: previous declaration is here
    DexMethod* m,
               ^
1 warning generated.

```
It's trivial to fix. So here it is.

Reviewed By: ssj933

Differential Revision: D17541194

fbshipit-source-id: 61f3d9220fd5d340cd81be0c60c2c792f695a173

facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex