[android-hack] Fix SleepEx() compilation under the Android NDK

Apparently the Android NDK on Wrench doesn't declare
clock_nanosleep()/etc., so 0947ffd8 resulted in the same error.

The correct fix would be to update the NDK on Wrench, which has a
proper solution which isn't yet ready, so in the meantime/for
expediency, declare the required functions so that we can get on with
our lives.

DO NOT CHERRY-PICK.