Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I2cf746fd07addc6e6c22e98e2a5bb8a8ce47c885
diff --git a/logd/LogBuffer.cpp b/logd/LogBuffer.cpp
index 8c30f79..db65978 100644
--- a/logd/LogBuffer.cpp
+++ b/logd/LogBuffer.cpp
@@ -380,7 +380,7 @@
             tid(tid),
             padding(0) {
     }
-    LogBufferElementKey(uint64_t key):value(key) { }
+    explicit LogBufferElementKey(uint64_t key):value(key) { }
 
     uint64_t getKey() { return value; }
 };