multifilesrc: fix caps leak
since gst_caps_replace()
and gst_pad_set_caps()
both ref the caps and neither of them takes the ownership of the caps -> it must be unreffed in gst_multi_file_src_set_property()
to test the leak (on Unix): echo coucou > /tmp/file.txt && GST_TRACERS=leaks GST_DEBUG="GST_TRACER:7" gst-launch-1.0 multifilesrc location=/tmp/file.txt caps='txt' ! fakesink