When building with gcc TSan on, and in Debug mode, we see a warning
like:

benchmark/src/timers.cc: In function ‘std::string benchmark::LocalDateTimeString()’:
src/timers.cc:241:15: warning: ‘char* strncat(char*, const char*, size_t)’ output may be truncated copying 108 bytes from a string of length 127 [-Wstringop-truncation]
  241 |   std::strncat(storage, tz_offset, sizeof(storage) - timestamp_len - 1);
      |   ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

While this is essentially a false positive (we never expect
the number of bytes in tz_offset to be too large), the compiler can't
actually tell that.  Shrink the size of tz_offset to a smaller, but still safe
size to eliminate this warning.
Signed-off-by: Chris Lalancette <clalancette@openrobotics.org>