ALSA: ppc: Add missing inclusion of linux/module.h

Otherwise it triggers a compile warning like:
 sound/ppc/keywest.c:104:1: warning: data definition has no type or storage class
 sound/ppc/keywest.c:104:1: error: type defaults to 'int' in declaration of 'MODULE_DEVICE_TABLE' [-Werror=implicit-int]

Fixes: a2bc2af6 ('ALSA: ppc: keywest: Export I2C module alias information')
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>