avr32: export strnlen_user

This patch fixes the following build error:

<--  snip  -->

...
  MODPOST 1327 modules
ERROR: "strnlen_user" [drivers/input/misc/uinput.ko] undefined!
...
make[2]: *** [__modpost] Error 1

<--  snip  -->
Reported-by: NAdrian Bunk <bunk@kernel.org>
Signed-off-by: NAdrian Bunk <bunk@kernel.org>
Signed-off-by: NHaavard Skinnemoen <haavard.skinnemoen@atmel.com>

arch/avr32/kernel/avr32_ksyms.c

@@ -43,6 +43,8 @@ EXPORT_SYMBOL(strncpy_from_user);
 EXPORT_SYMBOL(__strncpy_from_user);
 EXPORT_SYMBOL(clear_user);
 EXPORT_SYMBOL(__clear_user);
+EXPORT_SYMBOL(strnlen_user);
+
 EXPORT_SYMBOL(csum_partial);
 EXPORT_SYMBOL(csum_partial_copy_generic);