libutils: Include limits for std::numeric_limits::max

This fixes the following error:
core/libutils/String8.cpp:316:67: error: incomplete type 'std::numeric_limits<long long unsigned int>' used in nested name specifier
  316 |         if (static_cast<size_t>(n) > std::numeric_limits<size_t>::max() - 1 ||
      |                                                                   ^~~

Change-Id: I80cdd2442e2798c37e066e3fdeee7dd5ac34b669