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: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>