libstdc++: Include <stdint.h> in <bits/atomic_wait.h>

This is needed in order to compile it as a header-unit, which might be
desired because it's included by both <atomic> and <semaphore>.

libstdc++-v3/ChangeLog:

	* include/bits/atomic_wait.h: Include <stdint.h>.

(cherry picked from commit 6c8f2d3a)