playback: fix build error for 64bit Windows build by MinGW

playback: fix build error for 64bit Windows build by MinGW

Casting to gpointer from gulong generates the following warning with
64bit Windows target MinGW:

    gstplaybin2.c: In function 'pad_added_cb':
    gstplaybin2.c:3476:7: error: cast to pointer from integer of different size [-Werror=int-to-pointer-cast]
           (gpointer) group_id_probe_handler);
           ^
    cc1: all warnings being treated as errors

We should cast to guintptr from gulong before we cast to gpointer.

https://bugzilla.gnome.org/show_bug.cgi?id=754755