protobuf dependencies are missing on the clean machine

[vlntb]

Context of Change

This was discovered while configuring a clean Ubuntu machine to run rippled. Two dependencies to support protobuf types are missing: libprotobuf-dev protobuf-compiler. The update is only relevant for manually configuring the C++ development environment.

Type of Change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Refactor (non-breaking change that only restructures code)
• [ ] Performance (increase or change in throughput and/or latency)
• [ ] Tests (you added tests for code that already exists, or your new feature included in this PR)
• [x] Documentation update
• [ ] Chore (no impact to binary, e.g. .gitignore, formatting, dropping support for older tooling)
• [ ] Release

API Impact

• [ ] Public API: New feature (new methods and/or new fields)
• [ ] Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• [ ] libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• [ ] Peer protocol change (must be backward compatible or bump the peer protocol version)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 76.1%. Comparing base (63209c2) to head (3e51bbe). Report is 4 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #5156     +/-   ##
=========================================
- Coverage     76.1%   76.1%   -0.0%     
=========================================
  Files          762     762             
  Lines        61469   61469             
  Branches      8121    8122      +1     
=========================================
- Hits         46807   46803      -4     
- Misses       14662   14666      +4     

see 5 files with indirect coverage changes