mfd: fix build failures in recently added ab5500 code

These files had implicit dependencies on modular support
which now show up as build failures with the module cleanup
work merged to mainline.

Reported-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
diff --git a/drivers/mfd/ab5500-debugfs.c b/drivers/mfd/ab5500-debugfs.c
index 6be1fe6..43c0ebb 100644
--- a/drivers/mfd/ab5500-debugfs.c
+++ b/drivers/mfd/ab5500-debugfs.c
@@ -4,6 +4,7 @@
  * Debugfs support for the AB5500 MFD driver
  */
 
+#include <linux/export.h>
 #include <linux/debugfs.h>
 #include <linux/seq_file.h>
 #include <linux/mfd/ab5500/ab5500.h>