mtd: Add export.h for EXPORT_SYMBOL/THIS_MODULE where needed

These two common macros will be no longer present everywhere.
Call out the include needs of them explicitly where required.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
diff --git a/drivers/mtd/mtdsuper.c b/drivers/mtd/mtdsuper.c
index 16b02a1..89f8e66 100644
--- a/drivers/mtd/mtdsuper.c
+++ b/drivers/mtd/mtdsuper.c
@@ -14,6 +14,7 @@
 
 #include <linux/mtd/super.h>
 #include <linux/namei.h>
+#include <linux/export.h>
 #include <linux/ctype.h>
 #include <linux/slab.h>