alpha: forward declare struct pt_regs in processor.h

Removal of exec domains uncovered this new warning.  processor.h re-used
struct pt_regs from personality.h which is now gone.

  ./arch/alpha/include/asm/processor.h:47:33: warning: 'struct pt_regs' declared inside parameter list [enabled by default]
Signed-off-by: Richard Weinberger <richard@nod.at>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Matt Turner <mattst88@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/alpha/include/asm/processor.h

@@ -44,6 +44,7 @@ struct task_struct;
 extern unsigned long thread_saved_pc(struct task_struct *);
 
 /* Do necessary setup to start up a newly executed thread.  */
+struct pt_regs;
 extern void start_thread(struct pt_regs *, unsigned long, unsigned long);
 
 /* Free all resources held by a thread. */