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>

drivers/mfd/ab5500-debugfs.c

@@ -4,7 +4,7 @@
  * Debugfs support for the AB5500 MFD driver
  */
 
-#include <linux/export.h>
+#include <linux/module.h>
 #include <linux/debugfs.h>
 #include <linux/seq_file.h>
 #include <linux/mfd/ab5500/ab5500.h>