mfd: pm8921: Include missing linux/module.h

Include <linux/module.h> in order to fix the following errors.

drivers/mfd/pm8921-core.c:209:16: error: expected declaration specifiers or '...' before string constant
drivers/mfd/pm8921-core.c:210:20: error: expected declaration specifiers or '...' before string constant
drivers/mfd/pm8921-core.c:211:16: error: expected declaration specifiers or '...' before string constant
drivers/mfd/pm8921-core.c:212:14: error: expected declaration specifiers or '...' before string constant

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>