Error building sample_code.cpp on Linux: undefined reference to `hmac_keccak_hash' [1 XMR]

[woodser]

Building sample_code.cpp on Linux results in a build error:

undefined reference to `hmac_keccak_hash'

This issue requests fixing the build error on Linux. It builds successfully on macOS.

To build the sample code, modify CMakeLists.txt with set(BUILD_SAMPLE ON) and follow the instructions to build monero-cpp.

That will generate a binary: ./build/sample_code

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[woodser]

Related to #27

[SChernykh]

@woodser It builds for me with the changes in #76