ibus-engines.mozc builds fine with the newer protobuf, this should as well.

This fixes:

FAILED: obj/engine/engine.engine.o
clang++ -MMD -MF obj/engine/engine.engine.o.d -DOS_LINUX -DMOZC_BUILD -DCHANNEL_DEV -DENABLE_GTK_RENDERER -DNDEBUG -DQT_NO_DEBUG -DNO_LOGGING -DIGNORE_HELP_FLAG -DIGNORE_INVALID_FLAG -I/build/source/src -Igen -Igen/proto_out -Wall -Wno-char-subscripts -Wno-sign-compare -Wno-deprecated-declarations -Wwrite-strings -fPIC -fno-exceptions -fmessage-length=0 -fno-strict-aliasing -funsigned-char -include base/namespace.h -pipe -pthread -fno-omit-frame-pointer -fstack-protector --param=ssp-buffer-size=4 -Wtype-limits -O2 -Wno-deprecated -Wno-covered-switch-default -Wno-unnamed-type-template-args -Wno-c++11-narrowing -std=gnu++0x -std=gnu++0x  -c ../../engine/engine.cc -o obj/engine/engine.engine.o
In file included from ../../engine/engine.cc:30:
In file included from /build/source/src/engine/engine.h:33:
In file included from /nix/store/pcs8pq4a5rkym1hzibqz7da45fxkmig7-gcc-8.3.0/include/c++/8.3.0/memory:62:
In file included from /nix/store/pcs8pq4a5rkym1hzibqz7da45fxkmig7-gcc-8.3.0/include/c++/8.3.0/bits/stl_algobase.h:66:
/nix/store/pcs8pq4a5rkym1hzibqz7da45fxkmig7-gcc-8.3.0/include/c++/8.3.0/bits/stl_iterator_base_funcs.h:183:2: error: cannot decrement value of type 'mozc::ZeroQueryDict::iterator'
        --__i;
        ^ ~~~
/nix/store/pcs8pq4a5rkym1hzibqz7da45fxkmig7-gcc-8.3.0/include/c++/8.3.0/bits/stl_iterator_base_funcs.h:206:12: note: in instantiation of function template specialization 'std::__advance<mozc::ZeroQueryDict::iterator, long>' requested here
      std::__advance(__i, __d, std::__iterator_category(__i));
           ^
/nix/store/pcs8pq4a5rkym1hzibqz7da45fxkmig7-gcc-8.3.0/include/c++/8.3.0/bits/stl_algo.h:2137:9: note: in instantiation of function template specialization 'std::advance<mozc::ZeroQueryDict::iterator, long>' requested here
          std::advance(__middle, __half);
               ^
/nix/store/pcs8pq4a5rkym1hzibqz7da45fxkmig7-gcc-8.3.0/include/c++/8.3.0/bits/stl_algo.h:2190:19: note: in instantiation of function template specialization 'std::__equal_range<mozc::ZeroQueryDict::iterator, unsigned long, __gnu_cxx::__ops::_Iter_less_val, __gnu_cxx::__ops::_Val_less_iter>' requested here
      return std::__equal_range(__first, __last, __val,
                  ^
/build/source/src/prediction/zero_query_dict.h:213:17: note: in instantiation of function template specialization 'std::equal_range<mozc::ZeroQueryDict::iterator, unsigned long>' requested here
    return std::equal_range(begin(), end(), iter.index());
                ^
1 error generated.