headers_check fix: mn10300, setup.h

fix the following 'make headers_check' warnings:

  usr/include/asm-mn10300/setup.h:14: extern's make no sense in userspace
  usr/include/asm-mn10300/setup.h:15: extern's make no sense in userspace
Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>

arch/mn10300/include/asm/setup.h

@@ -11,7 +11,8 @@
 #ifndef _ASM_SETUP_H
 #define _ASM_SETUP_H
 
+#ifdef __KERNEL__
 extern void __init unit_setup(void);
 extern void __init unit_init_IRQ(void);
-
+#endif
 #endif /* _ASM_SETUP_H */