[MIPS] fix sparse warning about setup_early_printk()

This patch fixes the following sparse warning:

<<<<<<<<

arch/mips/kernel/early_printk.c:35:13: warning: symbol 'setup_early_printk'
was not declared. Should it be static?

<<<<<<<<

The fix is to define a prototype of the setup_early_printk() function and
to include the appropriate header into arch/mips/kernel/early_printk.c.

[Ralf: Sorted includes again]

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@movial.fi>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
diff --git a/arch/mips/kernel/setup.c b/arch/mips/kernel/setup.c
index c04e4e3..8af8486 100644
--- a/arch/mips/kernel/setup.c
+++ b/arch/mips/kernel/setup.c
@@ -550,11 +550,7 @@
 	prom_init();
 
 #ifdef CONFIG_EARLY_PRINTK
-	{
-		extern void setup_early_printk(void);
-
-		setup_early_printk();
-	}
+	setup_early_printk();
 #endif
 	cpu_report();
 	check_bugs_early();