Move PREEMPT_NOTIFIERS into an always-included Kconfig

Kconfig.preempt is not included on some archs (for example, m68k).  On those
archs, the Kconfig machinery complains that KVM selects an undefined symbol
PREEMPT_NOTIFIERS (which lives in Kconfig.preempt).

So move the offending symbol into a Kconfig file which is included by
everyone.

Cc: Roman Zippel <zippel@linux-m68k.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Avi Kivity <avi@qumranet.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

init/Kconfig

@@ -689,3 +689,6 @@ config STOP_MACHINE
 	  Need stop_machine() primitive.
 
 source "block/Kconfig"
+
+config PREEMPT_NOTIFIERS
+	bool

kernel/Kconfig.preempt

@@ -63,6 +63,3 @@ config PREEMPT_BKL
 	  Say Y here if you are building a kernel for a desktop system.
 	  Say N if you are unsure.
 
-config PREEMPT_NOTIFIERS
-	bool
-