db/VHelper: include DetachedSong.hxx to fix GCC 9 build failure

GCC 9's libstdc++ is unable to use forward-declared types as
std::vector item because the compiler wants to resolve `noexcept()` on
the item destructor.

src/db/VHelper.cxx

@@ -18,7 +18,6 @@
  */
 
 #include "VHelper.hxx"
-#include "song/DetachedSong.hxx"
 #include "song/LightSong.hxx"
 #include "song/Filter.hxx"
 

src/db/VHelper.hxx

@@ -22,6 +22,7 @@
 
 #include "Visitor.hxx"
 #include "Selection.hxx"
+#include "song/DetachedSong.hxx"
 
 #include <vector>