Fix cpplint build/namespaces issues

Change-Id: I19c68703270c1482d6c6aad8cdf97d3d2924360a
diff --git a/runtime/gc/collector/mark_sweep.cc b/runtime/gc/collector/mark_sweep.cc
index 2ca5f2d..2d07f062 100644
--- a/runtime/gc/collector/mark_sweep.cc
+++ b/runtime/gc/collector/mark_sweep.cc
@@ -50,7 +50,10 @@
 #include "thread_list.h"
 #include "verifier/method_verifier.h"
 
-using namespace art::mirror;
+using ::art::mirror::Class;
+using ::art::mirror::Field;
+using ::art::mirror::Object;
+using ::art::mirror::ObjectArray;
 
 namespace art {
 namespace gc {