add and fix -Wdeprecated-copy

*so* many warnings, from only two definitions

Change-Id: If2561cd500c05a1e33cce984faf9f3e42a8a95ac

src/libutil/fmt.hh

@@ -185,6 +185,8 @@ public:
 
     HintFmt(const HintFmt & hf) : fmt(hf.fmt) {}
 
+    HintFmt & operator=(HintFmt const & rhs) = default;
+
     std::string str() const
     {
         return fmt.str();

src/libutil/ref.hh

@@ -77,6 +77,8 @@ public:
         return ref<T2>((std::shared_ptr<T2>) p);
     }
 
+    ref<T> & operator=(ref<T> const & rhs) = default;
+
     bool operator == (const ref<T> & other) const
     {
         return p == other.p;