Add missing sched.h include for CLONE_NEWNS

This fixes the compile with `-stdlib=libc++`. Without this header the build fails with:

```
../src/cc/ns_guard.cc:75:30: error: use of undeclared identifier 'CLONE_NEWNS'
```

src/cc/ns_guard.cc

@@ -16,6 +16,7 @@
  */
 
 #include <fcntl.h>
+#include <sched.h>
 #include <sys/stat.h>
 #include <string>