Fix google-explicit-constructor warnings in system/vold.

Bug: 28341362
Change-Id: I14344b0cba134570bf87e7d61aa03503735dd794
Test: build with WITH_TIDY=1
diff --git a/EmulatedVolume.h b/EmulatedVolume.h
index 09686c1..9b0c049 100644
--- a/EmulatedVolume.h
+++ b/EmulatedVolume.h
@@ -37,7 +37,7 @@
  */
 class EmulatedVolume : public VolumeBase {
 public:
-    EmulatedVolume(const std::string& rawPath);
+    explicit EmulatedVolume(const std::string& rawPath);
     EmulatedVolume(const std::string& rawPath, dev_t device, const std::string& fsUuid);
     virtual ~EmulatedVolume();