kernel: fix kmod-sound-core build error

Fix build error introduced with 2ab6aac.

Signed-off-by: Mathias Kresin <[email protected]>

package/kernel/linux/modules/sound.mk

@@ -49,7 +49,7 @@ define KernelPackage/sound-core
 	CONFIG_SND_RAWMIDI \
 	CONFIG_SND_TIMER \
 	CONFIG_SND_PCM \
-	CONFIG_SND_PCM_TIMER \
+	CONFIG_SND_PCM_TIMER=y \
 	CONFIG_SND_SEQUENCER \
 	CONFIG_SND_VIRMIDI \
 	CONFIG_SND_SEQ_DUMMY \