Fix type precision warning in Xcode 9

[jonathonracz]

Nothing more than it says in the title. Xcode 9 was (rightfully) complaining about the std::int64_t data being downcast to an unsigned int (the type of the tv_sec field), so I silenced it with a static cast.

This is unlikely to cause issues since the usec-scale time is being divided by 1000000 to convert to sec.

[cameron314]

Thanks! :-)
Any idea why this is not a problem for tv_nsec as well?

[jonathonracz]

I would presume the constant 1000 multiplicand is being implicitly cast to tv_nsec's clock_res_t (int) type during the assignment of the result, and is also implicitly casting the (timeout_usecs % 1000000) by being a multiplicand.

Definitely not implicit casting clang wizardry I (or most people) would use naturally, but I guess it just so happened to work out.

[cameron314]

The right-hand expression is compiled independently of the assignment context, so the implicit cast to int must be done just before the assignment. I guess there's only a warning when mixing signed/unsigned types. MSVC would have warned here, I think. Hmm.