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/percpu.h b/include/linux/percpu.h
index ba2e85a..caebf2a 100644
--- a/include/linux/percpu.h
+++ b/include/linux/percpu.h
@@ -5,6 +5,7 @@
 #include <linux/preempt.h>
 #include <linux/smp.h>
 #include <linux/cpumask.h>
+#include <linux/printk.h>
 #include <linux/pfn.h>
 #include <linux/init.h>