[MIPS] don't leak setup_early_printk() in userspace header

Our userspace headers shouldn't contain prototypes of in-kernel
functions.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

include/asm-mips/setup.h

@@ -3,6 +3,8 @@
 
 #define COMMAND_LINE_SIZE	256
 
+#ifdef  __KERNEL__
 extern void setup_early_printk(void);
+#endif /* __KERNEL__ */
 
 #endif /* __SETUP_H */