sh64: re-add the __strnlen_user() prototype

Commit 42fd3b14
(sh: Initial consolidation of the _32/_64 uaccess split.)
mistakenly removed the sh64 __strnlen_user() prototype,
resulting in the following compile error:

<--  snip  -->

...
  CC      init/main.o
In file included from include/linux/poll.h:13,
                 from include/linux/rtc.h:113,
                 from include/linux/efi.h:19,
                 from init/main.c:43:
arch/sh/include/asm/uaccess.h: In function 'strnlen_user':
arch/sh/include/asm/uaccess.h:213: error: implicit declaration of function '__strnlen_user'
...
make[2]: *** [init/main.o] Error 1

<--  snip  -->

Reported-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>