Fix sn_force_interrupt_flag no defination issue

sn_force_interrupt_flag was removed in 39-rc1 patch. But it still be used in arch/ia64/sn/kernel/sn2/sn_proc_fs.c. that make compilation failed:
arch/ia64/sn/built-in.o: In function `sn_force_interrupt_show':
sn_proc_fs.c:(.text+0xd281): undefined reference to sn_force_interrupt_flag' sn_proc_fs.c:(.text+0xd2a0): undefined reference to sn_force_interrupt_flag' A quick fix for this.

---
 arch/ia64/sn/kernel/sn2/sn_proc_fs.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

index c76d8dc..ce3215d 100644
@@ -51,7 +51,7 @@ static int licenseID_open(struct inode *inode, struct file *file)
  * the bridge chip.  The hardware will then send an interrupt message if the
  * interrupt line is active.  This mimics a level sensitive interrupt.
  */
-extern int sn_force_interrupt_flag;
+int sn_force_interrupt_flag;
 
 static int sn_force_interrupt_show(struct seq_file *s, void *p)
 {