mfd: Add export.h for EXPORT_SYMBOL/THIS_MODULE as required

These macros are in <linux/export.h> and will no longer be
implicitly everywhere anymore.  Fix it up in advance.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
diff --git a/drivers/mfd/pcf50633-irq.c b/drivers/mfd/pcf50633-irq.c
index 1b0192f..048a3b9 100644
--- a/drivers/mfd/pcf50633-irq.c
+++ b/drivers/mfd/pcf50633-irq.c
@@ -15,6 +15,7 @@
 #include <linux/interrupt.h>
 #include <linux/kernel.h>
 #include <linux/mutex.h>
+#include <linux/export.h>
 #include <linux/slab.h>
 
 #include <linux/mfd/pcf50633/core.h>