Add header that defines strerror()

It helps to fix a compilation issue with host libc:

core/base/errors_unix.cpp:25:10: error: use of undeclared identifier 'strerror'
  return strerror(error_code);
         ^

Change-Id: I71d4221cc1bfa6f0cc764b40b6b28159994b8e04
1 file changed