printk/percpu: Define printk_func when printk is not defined

To avoid include hell, the per_cpu variable printk_func was declared
in percpu.h. But it is only defined if printk is defined.

As users of printk may also use the printk_func variable, it needs to
be defined even if CONFIG_PRINTK is not.

Also add a printk.h include in percpu.h just to be safe.

Link: http://lkml.kernel.org/r/20141121183215.01ba539c@canb.auug.org.au

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
diff --git a/include/linux/printk.h b/include/linux/printk.h
index 3bbd979..c69be9e 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -124,6 +124,8 @@
 void early_printk(const char *s, ...) { }
 #endif
 
+typedef int(*printk_func_t)(const char *fmt, va_list args);
+
 #ifdef CONFIG_PRINTK
 asmlinkage __printf(5, 0)
 int vprintk_emit(int facility, int level,
@@ -162,8 +164,6 @@
 
 extern void wake_up_klogd(void);
 
-typedef int(*printk_func_t)(const char *fmt, va_list args);
-
 void log_buf_kexec_setup(void);
 void __init setup_log_buf(int early);
 void dump_stack_set_arch_desc(const char *fmt, ...);