timer: Use a single definition of NSEC_PER_SEC for the whole codebase

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-id: c6e55468856ba0b8f95913c4da111cc0ef266541.1434113783.git.berto@igalia.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>