Move 'panic_timeout' to linux/kernel.h.

ipmi_watchdog.c wanted to know why panic_timeout isn't in some header file.
 However, ipmi_watchdog.c doesn't even use it, so that reference was
deleted.  Other references now use kernel.h instead of straight extern int.
Signed-off-by: Randy Dunlap <rddunlap@osdl.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>