md: Add in export.h for files using EXPORT_SYMBOL

These files were getting the defines for EXPORT_SYMBOL because
device.h was including module.h.  But we are going to put an
end to that.  So add the proper export.h include now.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>

drivers/md/dm-snap-transient.c

@@ -10,6 +10,7 @@
 #include <linux/mm.h>
 #include <linux/pagemap.h>
 #include <linux/vmalloc.h>
+#include <linux/export.h>
 #include <linux/slab.h>
 #include <linux/dm-io.h>