Allow nanosleep in preauth privsep child.

The new timing attack mitigation code uses nanosleep in the preauth
codepath, allow in systrace andbox too.

sandbox-systrace.c

@@ -59,6 +59,7 @@ static const struct sandbox_policy preauth_policy[] = {
 	{ SYS_getpgid, SYSTR_POLICY_PERMIT },
 	{ SYS_clock_gettime, SYSTR_POLICY_PERMIT },
 	{ SYS_gettimeofday, SYSTR_POLICY_PERMIT },
+	{ SYS_nanosleep, SYSTR_POLICY_PERMIT },
 	{ SYS_sigprocmask, SYSTR_POLICY_PERMIT },
 
 #ifdef SYS_getentropy