message: Fix uninitialize variable warning in murmur bench
  bench/murmur_hash.cpp:15:13: warning: variable 'x' is uninitialized when used here [-Wuninitialized]
  x += MurmurHash3(5 * 0xfba4c795, in);
  ^
  bench/murmur_hash.cpp:11:15: note: initialize the variable 'x' to silence this warning
  unsigned x;
  ^
  = 0
diff: static void Murmur3(benchmark::State &state)
  {
  std::vector<uint8_t> in(32, 0);
  - unsigned x;
  + unsigned int x = 0;
  while (state.KeepRunning())
  {
  for (int i = 0; i < 1000000; i++)
  