h8300: Fix missing consts in kernel_execve()

Fix missing consts in h8300's kernel_execve():

  arch/h8300/kernel/sys_h8300.c: In function 'kernel_execve':
  arch/h8300/kernel/sys_h8300.c:59: warning: initialization from incompatible pointer type
  arch/h8300/kernel/sys_h8300.c:60: warning: initialization from incompatible pointer type

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>