Added missing inclusions. Use std::ptrdiff_t.
(Thanks to Paulo Cesar Pereira de Andrade.)
This patch back-ported from
http://www.cs.unipr.it/git/gitweb.cgi?p=ppl/ppl.git;a=commit;h=61d4e14dfd9f1121e9b4521dead5728b2424dd7c
Change-Id: I0b11157dc52c66524c1035d54b69154aea1dace5
Signed-off-by: Chanho Park <chanho61.park@samsung.com>