main: use pdf::link::Destination copy constructor.

Fixes:

  main.cc: In function ‘int get_page_for_goto_link(pdf::link::GoTo*, pdf::Catalog*)’:
  main.cc:92:27: error: ‘const Destination’ {aka ‘const class LinkDest’} has no member named ‘copy’

https://cgit.freedesktop.org/poppler/poppler/commit/?id=7a429c3cf9fba67e

main.cc

@@ -89,7 +89,7 @@ static int get_page_for_goto_link(pdf::link::GoTo *goto_link, pdf::Catalog *cata
 #endif
   }
   else
-    dest.reset(orig_dest->copy());
+    dest.reset(new pdf::link::Destination(*orig_dest));
   if (dest.get() != nullptr)
   {
     int page;