Fix cpplint runtime/explicit issues

Change-Id: I352ba0b427f1ff9b22887693952b180eae0839ba
diff --git a/compiler/driver/dex_compilation_unit.h b/compiler/driver/dex_compilation_unit.h
index 53efd12..facc9cb 100644
--- a/compiler/driver/dex_compilation_unit.h
+++ b/compiler/driver/dex_compilation_unit.h
@@ -32,7 +32,7 @@
 
 class DexCompilationUnit {
  public:
-  DexCompilationUnit(CompilationUnit* cu);
+  explicit DexCompilationUnit(CompilationUnit* cu);
 
   DexCompilationUnit(CompilationUnit* cu, jobject class_loader, ClassLinker* class_linker,
                      const DexFile& dex_file, const DexFile::CodeItem* code_item,