FindProtobuf: Search for debug library named with `d` suffix

Protobuf now provides a CMake-based build system that optionally adds a
suffix to the debug library name [1].  Update our `find_library` call to
consider names with the default value for this suffix, `d`.

[1] https://github.com/google/protobuf/blob/431cee60/cmake/libprotoc.cmake#L107