In some reference libprotobuf/libprotoc installations the include
paths of the protobuf::libprotoc target are missing.
Force using protobuf::libprotobuf as the dependency for the
libprotoc compile test.
Change-Id: I56bb84910bc178063d42292f04d7e75c04e9ca4d
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Dennis Oberst <dennis.oberst@qt.io>
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>