tests: dash_mpd: add missing downloadrequest include

After updating my code base and run a ninja -C builddir, I got the error:

/usr/bin/ld: subprojects/gst-plugins-good/tests/check/elements_dash_mpd.p/elements_dash_mpd.c.o: in function `gst_mpd_client2_fetch_external_segment_list':
/myhome/gstreamer/builddir/../subprojects/gst-plugins-good/tests/check/../../ext/adaptivedemux2/dash/gstmpdclient.c:264: undefined reference to `download_request_take_buffer'
...

A first fix would be !2227 (closed) but could we plan to have a a mpdparser lib and a downloadhelper lib instead of including the c files in the test https://gitlab.freedesktop.org/gstreamer/gstreamer/-/blob/main/subprojects/gst-plugins-good/tests/check/elements/dash_mpd.c#L49 ?

I havent investigated neither why the CI did not complain about this ...

(Copied this over from !2117 (comment 1344646), tpm)