ASoC: SAMSUNG: Fix build error

This patch adds <linux/modules.h> to fix following build errors.

sound/soc/codecs/wm8994.c: In function 'wm8994_readable':
sound/soc/codecs/wm8994.c:58: warning: unused variable 'wm8994'
sound/soc/samsung/smdk_wm8994.c:176: error: expected declaration specifiers or '...' before string constant
sound/soc/samsung/smdk_wm8994.c:176: warning: data definition has no type or storage class
sound/soc/samsung/smdk_wm8994.c:176: warning: type defaults to 'int' in declaration of 'MODULE_DESCRIPTION'
sound/soc/samsung/smdk_wm8994.c:176: warning: function declaration isn't a prototype
sound/soc/samsung/smdk_wm8994.c:177: error: expected declaration specifiers or '...' before string constant

Signed-off-by: Boojin Kim <boojin.kim@samsung.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>