Move Stopwatch into its own header

Bug: 29748723
Change-Id: I7433f766909177b0ec945aa26f98534069b35891
Test: netd_integration_test
diff --git a/client/NetdClient.cpp b/client/NetdClient.cpp
index 392b0af..91d630d 100644
--- a/client/NetdClient.cpp
+++ b/client/NetdClient.cpp
@@ -26,6 +26,7 @@
 #include "FwmarkClient.h"
 #include "FwmarkCommand.h"
 #include "resolv_netid.h"
+#include "Stopwatch.h"
 
 namespace {