[CMAKE] Fix missing protobuf include directory
The Protobuf include directory has been mistakenly removed in D3016.
This patch reverts the previous wrong change.
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.
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