Update COPYING

Add missing copyright notice.

Fixes fontconfig/fontconfig#219
10 jobs for issue/219 in 7 minutes and 39 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #4731527
gst-macos-10.15
meson macos

00:01:07

passed #4731524
meson-mingw-w64-build

00:06:16

manual #4731528
docker windows 1809 allowed to fail manual
meson msys2
passed #4731522
meson-shared-build

00:06:24

passed #4731523
meson-static-build

00:06:32

passed #4731525
docker windows 1809
meson vs2017 amd64

00:01:24

passed #4731526
docker windows 1809
meson vs2017 x86

00:01:24

passed #4731521
mingw-build

00:06:31

passed #4731519
shared-build

00:07:39

passed #4731520
static-build

00:06:19