avutil: Add missing reference files for pixdesc fate test

Commit cd48318035 added support for NV24 and NV42, including several
fate tests for these formats, but did not include the reference files
for the tests filter-pixdesc-nv24 and filter-pixdesc-nv42. As a result,
these two tests were broken.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

tests/ref/fate/filter-pixdesc-nv24

@@ -0,0 +1 @@
+pixdesc-nv24        7437f36b6ee58050564b20a1f839ff07

tests/ref/fate/filter-pixdesc-nv42

@@ -0,0 +1 @@
+pixdesc-nv42        110bad2f58424ab800ad832f6966cafe