futex: add missing header guards

The header file was introduced by fbcc3e5 ("qemu-thread: optimize QemuLockCnt
with futexes on Linux", 2017-01-16) without header guards. Add them.

Signed-off-by: Emilio G. Cota <cota@braap.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
