Fix a bunch more lint.

(If you ignore the compilers, we've got relatively little lint now. Fits on
a single screen.)

Change-Id: I51389002894d4fd8cf46f79d2bac57079322a030
diff --git a/src/stack.h b/src/stack.h
index 9093cb1..a26b632 100644
--- a/src/stack.h
+++ b/src/stack.h
@@ -41,7 +41,7 @@
  public:
   Frame() : sp_(NULL) {}
 
-  Frame(Method** sp) : sp_(sp) {}
+  explicit Frame(Method** sp) : sp_(sp) {}
 
   Method* GetMethod() const {
     return (sp_ != NULL) ? *sp_ : NULL;