Fixed syntax on nfc_modulation declaration

Build was failing for me on:

```
  CXX(target) Release/obj.target/nfc/src/nfc.o
../src/nfc.cc:15:3: error: expected primary-expression before ‘.’ token
../src/nfc.cc:16:3: error: expected primary-expression before ‘.’ token
```

This fixes it for me, I'm not aware of any possible implications though?