Conditionally expose splice(2) and sync_file_range(2) constants.

As in other places, only expose the constants at API levels where
we're also exposing the functions (to cope with "poor man's configure"
where an author assumes the presence of the #define to imply the
availability of the function).

Bug: http://b/63145226
Test: builds
Change-Id: I524f7288513aa6c35479c550e748a619397929b0