ALSA: aloop: Fix dependency on timer API

An explicit Kconfig dependency is missing for the recent addition of
the timer support.  CONFIG_SND_TIMER isn't always selected by SND_PCM.

Fixes: 26c53379 ("ALSA: aloop: Support selection of snd_timer instead of jiffies")
Reported-by: kbuild test robot <lkp@intel.com>
Link: https://lore.kernel.org/r/20191124083924.14049-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/drivers/Kconfig

@@ -92,6 +92,7 @@ config SND_DUMMY
 config SND_ALOOP
 	tristate "Generic loopback driver (PCM)"
 	select SND_PCM
+	select SND_TIMER
 	help
 	  Say 'Y' or 'M' to include support for the PCM loopback device.
 	  This module returns played samples back to the user space using