Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I080090d50f1a368a4e7c1a0732297bb6a295e34f
diff --git a/tools/aapt2/unflatten/BinaryResourceParser.cpp b/tools/aapt2/unflatten/BinaryResourceParser.cpp
index ec46751..5cd1c8b 100644
--- a/tools/aapt2/unflatten/BinaryResourceParser.cpp
+++ b/tools/aapt2/unflatten/BinaryResourceParser.cpp
@@ -47,7 +47,7 @@
 public:
     using ValueVisitor::visit;
 
-    ReferenceIdToNameVisitor(const std::map<ResourceId, ResourceName>* mapping) :
+    explicit ReferenceIdToNameVisitor(const std::map<ResourceId, ResourceName>* mapping) :
             mMapping(mapping) {
         assert(mMapping);
     }