libnaio: Fix c++11 narrowing

Summary:
Fixes the following:
error: non-constant-expression cannot be narrowed from
type 'uint32_t' (aka 'unsigned int') to 'long' in initializer list

const struct timespec req = {0, ns};
                                ^~

Test Plan: verified

Reviewers: randall.rushing, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D631