kselftest/alsa: Add a .gitignore for the newly added PCM test

The newly added PCM test produces a binary which is not ignored by git
when built in tree, fix that.

Fixes: aba51cd0 ("selftests: alsa - add PCM test")
Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20221125153654.1037868-1-broonie@kernel.orgSigned-off-by: Takashi Iwai <tiwai@suse.de>

tools/testing/selftests/alsa/.gitignore

 mixer-test
+pcm-test