rt: Fix build error with !CONFIG_PREEMPT_RT

Here's a small build fix to get !CONFIG_PREEMPT_RT +
CONFIG_PREEMPT_HARDIRQS to build. Not sure why this change was
introduced between 2.6.29-rt and 2.6.31-rt, but this sets it back.

Tested with and without CONFIG_PREEMPT_RT and with and without
CONFIG_PREEMPT_HARDIRQS.
Signed-off-by: John Stultz <johnstul@us.ibm.com>
Cc: Clark Williams <williams@redhat.com>
Cc: Steven Rostedt <srostedt@redhat.com>
Cc: Darren Hart <dvhltc@us.ibm.com>
LKML-Reference: <1249428652.3289.19.camel@localhost>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

include/linux/bottom_half.h

 #ifndef _LINUX_BH_H
 #define _LINUX_BH_H
 
-#ifdef CONFIG_PREEMPT_RT
+#ifdef CONFIG_PREEMPT_HARDIRQS
 # define local_bh_disable()		do { } while (0)
 # define __local_bh_disable(ip)		do { } while (0)
 # define _local_bh_enable()		do { } while (0)