(error) Same iterator is used with different containers

cppcheck reports this error, its not really a problem for us as the API
can actually deal with it via implicit conversion, but being explicit
can't hurt and the less reported errors the better.

Git-Dch: Ignore

cmdline/apt-mark.cc

@@ -279,7 +279,7 @@ static bool DoHold(CommandLine &CmdL)
    {
       if (Pkg->CurrentVer != 0)
 	 continue;
-      keepoffset.insert(Pkg);
+      keepoffset.insert(*Pkg);
    }
 
    if (keepoffset.empty() == false)