[boost] [1.58.0] Request permission to merge smart_ptr:94824c8 into master

This https://github.com/boostorg/smart_ptr/commit/94824c807f76dc4f3d41109a75808ed... fixes a problem on QNX/Blackberry by adding std:: to ptrdiff_t, in a place where size_t is already similarly qualified, so it should be very low risk. Tested locally on g++ 4.8.3, clang-3.4.2, msvc-8.0, msvc-10.0, msvc-11.0, msvc-12.0.