h8300/uaccess: add mising __clear_user()

Fix the build error:

  include/linux/regset.h: In function 'user_regset_copyout_zero':
  include/linux/regset.h:289:3: error: implicit declaration of function '__clear_user' [-Werror=implicit-function-declaration]

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>