Fix cpplint runtime/explicit issues

Change-Id: I352ba0b427f1ff9b22887693952b180eae0839ba
diff --git a/runtime/base/mutex.cc b/runtime/base/mutex.cc
index bb4b5c5..25c0b9e 100644
--- a/runtime/base/mutex.cc
+++ b/runtime/base/mutex.cc
@@ -94,7 +94,7 @@
 
 class ScopedAllMutexesLock {
  public:
-  ScopedAllMutexesLock(const BaseMutex* mutex) : mutex_(mutex) {
+  explicit ScopedAllMutexesLock(const BaseMutex* mutex) : mutex_(mutex) {
     while (!all_mutexes_guard_.CompareAndSwap(0, reinterpret_cast<int32_t>(mutex))) {
       NanoSleep(100);
     }