Allow nanosleep in preauth privsep child.

The new timing attack mitigation code uses nanosleep in the preauth
codepath, allow in sandbox.

sandbox-seccomp-filter.c

@@ -193,6 +193,9 @@ static const struct sock_filter preauth_insns[] = {
 #ifdef __NR_munmap
 	SC_ALLOW(__NR_munmap),
 #endif
+#ifdef __NR_nanosleep
+	SC_ALLOW(__NR_nanosleep),
+#endif
 #ifdef __NR__newselect
 	SC_ALLOW(__NR__newselect),
 #endif