watchdog/core: Add missing prototypes for weak functions

commit 81bd415c upstream.

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: 73ce0511 ("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.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/nmi.h

@@ -119,6 +119,8 @@ static inline int hardlockup_detector_perf_init(void) { return 0; }
 void watchdog_nmi_stop(void);
 void watchdog_nmi_start(void);
 int watchdog_nmi_probe(void);
+int watchdog_nmi_enable(unsigned int cpu);
+void watchdog_nmi_disable(unsigned int cpu);
 
 /**
  * touch_nmi_watchdog - restart NMI watchdog timeout.