hls: fix for assert failure when using encrypted HLS streams

hls: fix for assert failure when using encrypted HLS streams

When using an HLS encrypted stream, an assertion failure is thrown:
(gst-launch-1.0:31028): GLib-GObject-WARNING **: cannot register
existing type `GstFragment'

(gst-launch-1.0:31028): GLib-CRITICAL **: g_once_init_leave: assertion
`result != 0' failed

Eventually tracked this down to the call gst_fragment_new()
in function gst_hls_demux_decrypt_fragment.

The GstFragment class is defined in ext/hls/gstfragment.c and in
gst-libs/gst/uridownloader/gstfragment.c. Having two class definitions
with the same name causes the assert failure when trying to allocate
GstFragment. Deleting the version from hls and editing the
Makefile.am solves this assert failure.

https://bugzilla.gnome.org/show_bug.cgi?id=704555