#6 error: 'ptrdiff_t' does not name a type

[MartinKG]

On Fedora 23 the following error occurs:
tree.h:118:21: error: 'ptrdiff_t' does not name a type
typedef ptrdiff_t difference_type;

Please commit the attached patch.

[Davi de Castro Reis]

Thanks a lot! Added in git head.