powerpc: Add missing prototype for arch_dup_task_struct()

In commit 55ccf3fe ("fork: move the real prepare_to_copy() users to
arch_dup_task_struct()") a new arch_dup_task_struct() was added
without a prototype declared in thread_info.h header. Fix the
following warning (treated as error in W=1):

  arch/powerpc/kernel/process.c:1609:5: error: no previous prototype for ‘arch_dup_task_struct’
Signed-off-by: NMathieu Malaterre <malat@debian.org>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/thread_info.h

@@ -70,6 +70,7 @@ static inline struct thread_info *current_thread_info(void)
 	return (struct thread_info *)val;
 }
 
+extern int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src);
 #endif /* __ASSEMBLY__ */
 
 /*