message: Add guard around definition of ssize_t
diff: #include <functional>
  #ifdef _MSC_VER
  +
  #if REALM_UWP
  struct timeval {
  long tv_sec;
  long tv_usec;
  };
  #endif
  +
  #include <windows.h>
  #include <intrin.h>
  -#endif
  -
  -#if defined(_MSC_VER)
  #include <BaseTsd.h>
  +
  +#if !defined(_SSIZE_T_) && !defined(_SSIZE_T_DEFINED)
  typedef SSIZE_T ssize_t;
  +#define _SSIZE_T_
  +#define _SSIZE_T_DEFINED
  #endif
  +#endif // _MSC_VER
  +
  #include <realm/util/features.h>
  #include <realm/util/assert.hpp>
  #include <realm/util/safe_int_ops.hpp>
  