[CMAKE] Fix missing protobuf include directory
[CMAKE] Fix missing protobuf include directory

Summary:
The Protobuf include directory has been mistakenly removed in D3016.

This patch reverts the previous wrong change.

Full explanation:
The include directories are automatically added by CMake when using
`target_link_libraries` on `IMPORTED` target, but this is not the case
here. `IMPORTED` targets have been added for protobuf in later CMake
versions, past our minimum required one.

Test Plan:
  mkdir buildcmake && cd buildcmake
  cmake -GNinja ..
  ninja
  ./src/qt/bitcoin-qt # Sanity check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3030

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