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>

include/qemu/futex.h

@@ -11,6 +11,9 @@
  *
  */
 
+#ifndef QEMU_FUTEX_H
+#define QEMU_FUTEX_H
+
 #include <sys/syscall.h>
 #include <linux/futex.h>
 
@@ -34,3 +37,5 @@ static inline void qemu_futex_wait(void *f, unsigned val)
         }
     }
 }
+
+#endif /* QEMU_FUTEX_H */