avcodec/tests: Add codec_desc to .gitignore

The compiled libavcodec/tests/codec_desc was left out of that dir's
.gitignore when the test was added, so it shows up in 'git status'
as an untracked file if it's been built.

Signed-off-by: FeRD (Frank Dana) <ferdnyc@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/tests/.gitignore

@@ -2,6 +2,7 @@
 /avpacket
 /cabac
 /celp_math
+/codec_desc
 /dct
 /fft
 /fft-fixed