Fix cpplint runtime/explicit issues

Change-Id: I352ba0b427f1ff9b22887693952b180eae0839ba
diff --git a/runtime/jdwp/jdwp_adb.cc b/runtime/jdwp/jdwp_adb.cc
index 9652f60..9b9fe4c 100644
--- a/runtime/jdwp/jdwp_adb.cc
+++ b/runtime/jdwp/jdwp_adb.cc
@@ -54,7 +54,7 @@
 
 struct JdwpAdbState : public JdwpNetStateBase {
  public:
-  JdwpAdbState(JdwpState* state) : JdwpNetStateBase(state) {
+  explicit JdwpAdbState(JdwpState* state) : JdwpNetStateBase(state) {
     control_sock_ = -1;
     shutting_down_ = false;