ALSA: pcmtest: Add missing module descriptions

Now that make W=1 starts complaining the lack of MODULE_DESCRIPTION(),
let's add the missing information.
Reported-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Closes: https://lore.kernel.org/r/ZjpQm-hxLQtpgkUx@smile.fi.intel.comSigned-off-by: Takashi Iwai <tiwai@suse.de>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Dragan Simic <dsimic@manjaro.org>
Link: https://lore.kernel.org/r/20240508091128.25274-6-tiwai@suse.de

sound/drivers/pcmtest.c

@@ -772,6 +772,7 @@ static void __exit mod_exit(void)
 	platform_device_unregister(&pcmtst_pdev);
 }
 
+MODULE_DESCRIPTION("Virtual ALSA driver for PCM testing/fuzzing");
 MODULE_LICENSE("GPL");
 MODULE_AUTHOR("Ivan Orlov");
 module_init(mod_init);