include/linux/sysctl.h: fix register_sysctl_mount_point() return type

The CONFIG_SYSCTL=n stub returns the wrong type.

Fixes: ee9efac4

 ("sysctl: add helper to register a sysctl mount point")
Reported-by: kernel test robot <lkp@intel.com>
Acked-by: Luis Chamberlain <mcgrof@kernel.org>
Cc: Tong Zhang <ztong0001@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>