DetachedSong: don't declare empty destructor

An explicit destructor prevents usage of implicit move operators, even
if it's empty.  Therefore, declaring a defaulted destructor with GCC
attribute "noinline" does what we want without preventing those
implicit operators.

src/DetachedSong.cxx

@@ -31,11 +31,6 @@ DetachedSong::DetachedSong(const LightSong &other)
 	 start_time(other.start_time),
 	 end_time(other.end_time) {}
 
-DetachedSong::~DetachedSong()
-{
-	/* this destructor exists here just so it won't  inlined */
-}
-
 bool
 DetachedSong::IsRemote() const
 {

src/DetachedSong.hxx

@@ -98,7 +98,8 @@ public:
 	 */
 	explicit DetachedSong(const LightSong &other);
 
-	~DetachedSong();
+	gcc_noinline
+	~DetachedSong() = default;
 
 	/* these are declared because the user-defined destructor
 	   above prevents them from being generated implicitly */