Add line between licenses in COPYING.

More clearly delineate which files are associated with each license.
12 jobs for master in 12 minutes and 20 seconds (queued for 1 second)
Status Name Job ID Coverage
  Build
passed meson macos #6032907
gst-macos-10.15

00:03:11

passed meson-mingw-w64-build #6032791

00:04:43

manual meson msys2 #6032795
docker windows 1809 allowed to fail manual
passed meson-shared-build #6032789

00:05:02

passed meson-static-build #6032790

00:05:27

passed meson vs2017 amd64 #6032792
docker windows 1809

00:01:22

passed meson vs2017 x86 #6032793
1809 docker windows

00:01:21

passed mingw-build #6032788

00:05:58

passed shared-build #6032908

00:06:21

passed static-build #6032787

00:05:30

failed meson macos #6032794
gst-macos-10.15

00:00:46

failed shared-build #6032786

00:05:09