coverity: add move operations

CID 1521171: Missing move assignment operator

Signed-off-by: Rosen Penev <rosenp@gmail.com>

include/exiv2/tags.hpp

@@ -308,6 +308,9 @@ class EXIV2API ExifKey : public Key {
   ~ExifKey() override;
   //@}
 
+  ExifKey(ExifKey&& rhs);
+  ExifKey& operator=(ExifKey&& rhs);
+
   //! @name Manipulators
   //@{
   /*!

src/tags.cpp

@@ -264,6 +264,9 @@ ExifKey::ExifKey(const std::string& key) : p_(std::make_unique<Impl>()) {
 ExifKey::ExifKey(const ExifKey& rhs) : p_(std::make_unique<Impl>(*rhs.p_)) {
 }
 
+ExifKey::ExifKey(ExifKey&& rhs) : p_(std::make_unique<Impl>(*rhs.p_)) {
+}
+
 ExifKey::~ExifKey() = default;
 
 ExifKey& ExifKey::operator=(const ExifKey& rhs) {
@@ -274,6 +277,14 @@ ExifKey& ExifKey::operator=(const ExifKey& rhs) {
   return *this;
 }
 
+ExifKey& ExifKey::operator=(ExifKey&& rhs) {
+  if (this == &rhs)
+    return *this;
+  *p_ = *rhs.p_;
+  Key::operator=(std::move(rhs));
+  return *this;
+}
+
 void ExifKey::setIdx(int idx) {
   p_->idx_ = idx;
 }