frv: export empty_zero_page

Fix the following build error:

ERROR: "empty_zero_page" [fs/ext4/ext4dev.ko] undefined!
Reported-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Adrian Bunk <bunk@kernel.org>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/frv/mm/init.c

@@ -26,6 +26,7 @@
 #include <linux/types.h>
 #include <linux/bootmem.h>
 #include <linux/highmem.h>
+#include <linux/module.h>
 
 #include <asm/setup.h>
 #include <asm/segment.h>
@@ -56,7 +57,9 @@ DEFINE_PER_CPU(struct mmu_gather, mmu_gathers);
  */
 static unsigned long empty_bad_page_table;
 static unsigned long empty_bad_page;
+
 unsigned long empty_zero_page;
+EXPORT_SYMBOL(empty_zero_page);
 
 /*****************************************************************************/
 /*