util: Move the implementation of futex_wake and futex_wait from futex.h to futex.c

util: Move the implementation of futex_wake and futex_wait from futex.h to futex.c

Doing this is to avoid include bloat on Windows that pulls windows.h,
for other unix platforms, it's also benefit include the used symbols only when use
futex_wake and futex_wait.
No functional change, there is no performance incur because function call is consume
much less time than syscall

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19120>