ptrdiff_t in recursive_directory_iterator should be std::ptrdiff_t

[unravel-dev]

Hi it's me again. Another issue i forgot to mention that is breaking the build is

class resursive_directory_iterator
{
...
using difference_type = ptrdiff_t;

should be 

using difference_type = std::ptrdiff_t;

[gulrak]

Oops... indeed.