Fix build.

cpplint errors.

Change-Id: I11141f6e19b56a3f53d6e52f3013f4d20f33f102
diff --git a/compiler/dex/frontend.h b/compiler/dex/frontend.h
index b6e8577..4a863f5 100644
--- a/compiler/dex/frontend.h
+++ b/compiler/dex/frontend.h
@@ -115,7 +115,7 @@
 
 class QuickCompilerContext {
   public:
-    QuickCompilerContext(CompilerDriver& compiler);
+    explicit QuickCompilerContext(CompilerDriver& compiler);
     ~QuickCompilerContext();
 
     DexFileToMethodInlinerMap* GetInlinerMap() {