kernel/watchdog.c: Use proper ANSI C prototypes

We try to enforce it by using -Wstrict-prototypes, but apparently they
sometimes get through.  Introduced by 4eec42f3 ("watchdog: Change
the default timeout and configure nmi watchdog period based").
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/watchdog.c

@@ -98,7 +98,7 @@ __setup("nosoftlockup", nosoftlockup_setup);
  * the thresholds with a factor: we make the soft threshold twice the amount of
  * time the hard threshold is.
  */
-static int get_softlockup_thresh()
+static int get_softlockup_thresh(void)
 {
 	return watchdog_thresh * 2;
 }