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

These two basic defines were everywhere, simply because module.h
was also everywhere.   But we are cleaning up the latter.  So make
the exporters actually call out their need for the include.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
diff --git a/drivers/mmc/core/debugfs.c b/drivers/mmc/core/debugfs.c
index 6045ea4..3923880 100644
--- a/drivers/mmc/core/debugfs.c
+++ b/drivers/mmc/core/debugfs.c
@@ -8,6 +8,7 @@
  * published by the Free Software Foundation.
  */
 #include <linux/moduleparam.h>
+#include <linux/export.h>
 #include <linux/debugfs.h>
 #include <linux/fs.h>
 #include <linux/seq_file.h>