Revert "DetachedSong: remove explicitly-defaulted copy/move constructors"

This reverts commit 67b7d46432.

Turns out I was wrong, and mentioning these does make a difference:
the implicit move constructor is not defined in the presence of a
user-declared destructor.

src/DetachedSong.hxx

@@ -100,6 +100,11 @@ public:
 
 	~DetachedSong();
 
+	/* these are declared because the user-defined destructor
+	   above prevents them from being generated implicitly */
+	explicit DetachedSong(const DetachedSong &) = default;
+	DetachedSong(DetachedSong &&) = default;
+
 	gcc_pure
 	const char *GetURI() const {
 		return uri.c_str();