microblaze: Fix ret_from_fork declaration

ret_from_fork is used by noMMU system too.

It should be the part of patch
"Disintegrate asm/system.h for Microblaze"
(sha1: c40d04df152a1111c5bbcb632278394dabd2b73d)

Signed-off-by: Michal Simek <monstr@monstr.eu>

arch/microblaze/include/asm/processor.h

@@ -31,6 +31,8 @@ extern const struct seq_operations cpuinfo_op;
 /* Do necessary setup to start up a newly executed thread. */
 void start_thread(struct pt_regs *regs, unsigned long pc, unsigned long usp);
 
+extern void ret_from_fork(void);
+
 # endif /* __ASSEMBLY__ */
 
 # ifndef CONFIG_MMU
@@ -143,8 +145,6 @@ static inline void exit_thread(void)
 
 unsigned long get_wchan(struct task_struct *p);
 
-extern void ret_from_fork(void);
-
 /* The size allocated for kernel stacks. This _must_ be a power of two! */
 # define KERNEL_STACK_SIZE	0x2000