partition_tools: Include string.h for strerror in lpadd

This fixes the following error:

extras/partition_tools/lpadd.cc:81:26: error: ‘strerror’ was not declared in this scope; did you mean ‘stderr’?
   81 |                       << strerror(saved_errno) << std::endl;
      |                          ^~~~~~~~
      |                          stderr

Change-Id: I05da9a447be4024943caab73bdc38776d9caa40e
1 file changed