[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>

arch/mips/kernel/early_printk.c

@@ -10,6 +10,8 @@
 #include <linux/console.h>
 #include <linux/init.h>
 
+#include <asm/setup.h>
+
 extern void prom_putchar(char);
 
 static void __init