watchdog/core: remove sysctl handlers from public header

The functions are only used in the file where they are defined.  Remove
them from the header and make them static.

Also guard proc_soft_watchdog with a #define-guard as it is not used
otherwise.

Link: https://lkml.kernel.org/r/20240306-const-sysctl-prep-watchdog-v1-1-bd45da3a41cf@weissschuh.net


Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>