watchdog/core: Add missing prototypes for weak functions

watchdog/core: Add missing prototypes for weak functions

The split out of the hard lockup detector exposed two new weak functions,
but no prototypes for them, which triggers the build warning:

  kernel/watchdog.c:109:12: warning: no previous prototype for ‘watchdog_nmi_enable’ [-Wmissing-prototypes]
  kernel/watchdog.c:115:13: warning: no previous prototype for ‘watchdog_nmi_disable’ [-Wmissing-prototypes]

Add the prototypes.

Fixes: 73ce0511c436 ("kernel/watchdog.c: move hardlockup detector to separate file")
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Babu Moger <babu.moger@oracle.com>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20180606194232.17653-1-malat@debian.org