Add missing 'explicit's on single-argument constructors.

Change-Id: I1494df6e74ec16238971fb4346ba184eb61c37ab
diff --git a/src/mutex.h b/src/mutex.h
index eb90565..a276b5d 100644
--- a/src/mutex.h
+++ b/src/mutex.h
@@ -81,7 +81,7 @@
 
 class ConditionVariable {
  public:
-  ConditionVariable(const std::string& name);
+  explicit ConditionVariable(const std::string& name);
   ~ConditionVariable();
 
   void Broadcast();