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 <[e-mail]>
Cc: Richard Henderson <[e-mail]>
Cc: Ivan Kokshaysky <[e-mail]>
Cc: Matt Turner <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/alpha/include/asm/processor.h

···
 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. */