selftests: ALSA: Add test-pcmtest-driver to .gitignore

It was forgotten to add the new binary to .gitignore.  Let's fix it.

Fixes: 10b98a4d ("selftests: ALSA: Add test for the 'pcmtest' driver")
Link: https://lore.kernel.org/r/20230719114336.18409-1-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>