[Chronik] Add ENABLE_CHRONIK #define macro

Summary: This is required for conditional compilation in the C++ code depending on whether Chronik is enabled.

Test Plan:

1. cmake with -DBUILD_BITCOIN_CHRONIK=ON generates #define ENABLE_CHRONIK 1 in build/src/config/bitcoin-config.h
2. cmake with -DBUILD_BITCOIN_CHRONIK=OFF generates #define ENABLE_CHRONIK 0 in build/src/config/bitcoin-config.h

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11806