nmi_watchdog: Only enable on x86 for now

It wont even build on other platforms just yet - so restrict it
to x86 for now.

Cc: Don Zickus <dzickus@redhat.com>
Cc: gorcunov@gmail.com
Cc: aris@redhat.com
Cc: peterz@infradead.org
LKML-Reference: <1265424425-31562-4-git-send-email-dzickus@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

lib/Kconfig.debug

@@ -173,6 +173,7 @@ config DETECT_SOFTLOCKUP
 config NMI_WATCHDOG
 	bool "Detect Hard Lockups with an NMI Watchdog"
 	depends on DEBUG_KERNEL && PERF_EVENTS
+	depends on X86
 	default y
 	help
 	  Say Y here to enable the kernel to use the NMI as a watchdog