[chronik] Install the protobuf file

Summary: This is a useful file to be distributed/installed as part of the standard installation.

Test Plan:

cmake -GNinja .. -DBUILD_BITCOIN_CHRONIK=ON -DCMAKE_INSTALL_PREFIX=install
ninja install

Check there is an install/proto/chronik.proto file installed.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14043