[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