Turn unknown attributes into warning

As far as I can tell, fontconfig can recover from encountering an unknown attribute.
And unknown elements already print a warning instead of an error, so let's make this a warning too.
10 jobs for unknown-attr-warn in 7 minutes and 6 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build
passed #4129570
gst-macos-10.15
meson macos

00:00:50

passed #4129567
meson-mingw-w64-build

00:02:46

manual #4129571
docker windows 1809 allowed to fail manual
meson msys2
passed #4129565
meson-shared-build

00:04:12

passed #4129566
meson-static-build

00:04:12

passed #4129568
docker windows 1809
meson vs2017 amd64

00:01:19

passed #4129569
docker windows 1809
meson vs2017 x86

00:01:11

passed #4129564
mingw-build

00:05:47

passed #4129562
shared-build

00:07:06

passed #4129563
static-build

00:05:32