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.de


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