Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I080090d50f1a368a4e7c1a0732297bb6a295e34f
diff --git a/tools/aapt2/link/ManifestFixer.cpp b/tools/aapt2/link/ManifestFixer.cpp
index 953e87e..4d8c5fb 100644
--- a/tools/aapt2/link/ManifestFixer.cpp
+++ b/tools/aapt2/link/ManifestFixer.cpp
@@ -220,7 +220,7 @@
 public:
     using xml::Visitor::visit;
 
-    FullyQualifiedClassNameVisitor(const StringPiece16& package) : mPackage(package) {
+    explicit FullyQualifiedClassNameVisitor(const StringPiece16& package) : mPackage(package) {
     }
 
     void visit(xml::Element* el) override {