mfd: include linux/module.h for ab5500-debugfs

Include linux/module.h to fix below build error:

  CC      drivers/mfd/ab5500-debugfs.o
drivers/mfd/ab5500-debugfs.c:571: error: 'THIS_MODULE' undeclared here (not in a function)
make[2]: *** [drivers/mfd/ab5500-debugfs.o] Error 1

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>