City hash android fix

The build breaks on building for android
```
Compile++ thumb  : ppsspp_jni <= ArmEmitter.cpp
../native/ext/cityhash/city.cpp: In function 'uint64 HashLen33to64(char const*, size_t)':
../native/ext/cityhash/city.cpp:335:14: error: 'UINT64_C' was not declared in this scope
```

So I redefined UNIT64_C (as it's done in __sceNet.h)